Oh boy, we've got a question I can answer. Let's start by clarifying your question, though. I'll frontload that my expertise is in the Latin liturgy, so I can't really speak to what Greek, Coptic, or Maronite Catholics would recognize.

If we're going to drop a contemporary Roman Catholic into the fourth or fifth century, I assume we'd like to drop him somewhere analogously Christian. Let's call it Rome? This works not just because there were Roman Christians in Rome (although I don't believe that they were particularly attached to the moniker "Catholic" yet), but because we've got plenty of sources for the Roman liturgy during this period.

On the topic of this period, the post-Constantinian but pre-Carolingian years are sort of a golden period for a certain generation of liturgical scholars who see it as the apogée of Christian worship, so a whole lot of ink has been spilt on this topic. This assumption informed the Catholic liturgical reforms of the 1960s-70s, so I'm sure that some liturgists would insist that a contemporary Catholic would recognize all sorts of things from the late antique/early medieval Latin liturgy. However, these scholars typically approached the liturgy from a pretty theological perspective, and have been more concerned with the perspectives of the Church Fathers or other important theologians than with the lived experience of Christian worshippers. Dom Gregory Dix's The Shape of the Liturgy is probably the best exemplar of this sort of work. The emphasis on clerical elites' perspectives makes sense - educated churchmen are by and large the people who have left us written sources, and on a certain level you have to work with what you've got. Other scholars have given more attention to the lay experience, especially in recent years. Susan Harvey has worked a lot on early Syriac Christian practice, especially women's practices; Teresa Berger has focused on female liturgical praxis as well. However, it should be noted that a lot of the scholarly interest in lay Christian practice focuses on the first centuries of Christianity, before the presumed clericalization of worship, which places this work a little bit before the timeframe you're asking about. My own work is in later medieval religious practice, which is a burgeoning field, but let's keep it within the centuries straddling the late antique and early medieval periods - a slight adjustment of your stated timeframe, which I hope you don't mind.

Historiography out of the way, let's talk about what Christians might recognize and what might seem foreign. The Mass seems like a decent enough place to begin. A lot of the clerical garb looked similar to what clergy wear today - the pope (bishop of Rome, who had a huge liturgical presence in the city of Rome for reasons we'll touch on below) would have worn a pretty ample chasuble, which by this point had ceased to be a secular garment and was a purely liturgical vestment, as well as a dalmatic, a deacon's sleeved garment, under this. This, too, had stopped being secular clothing. And of course the pope wore a band of wool around his neck called a pallium, a symbol of episcopal authority that originally in the West was only worn by the pope, but later became the garment of all metropolitan bishops. However, the mitre wasn't yet in use, nor the zucchetto (a bishop's skullcap - today the pope's is white), so the pope would have been bareheaded. I'm also not sure if he would have been wearing a stole. The ferula, which is the tall cross that the pope carries in processions (analogous to a bishop's crosier) was also not used - in fact, unlike other Latin bishops, the pope would not carry any staff at all. This is because the pope had both his arms carried by deacons during processions, which is an old Roman honorific practice. Today's Catholics would recognize the use of incense at Mass, although we have artistic evidence that the chains on censers were shorter during this period, so they would have been swung in something more like contemporary Greek fashion - full chain swings, none of this gentle contemporary Latin wafting incense about gently.

The actual order of Mass was largely in place by this point, so Catholics transported back to fifth-century Rome might recognize a liturgical skeleton. The ministers would process in through the middle of the church, although probably not to music in most cases. Scholars like Anton Baumstark and Robert Taft have observed that a lot of liturgical music sprung up to fill "soft spots" in the liturgy like processions. However, Josef Jungmann argues that singing an introit Psalm was not typical, and reserved for special occasions in the Roman liturgy, until the Carolingians misunderstood the Roman ordo and prescribed an introit for every Mass. Regardless, the ministers would process to the altar and prostrate themselves before it. Today, clergy bow and kiss the altar, but in the past prostrations have been more elaborate. Catholics might recognize this prostration because it has survived in the Good Friday liturgy. Another sung bit of the Mass that today's Catholics would notice the absence of is the Kyrie, the Greek invocation "Lord have mercy, Christ have mercy, Lord have mercy." This actually was brought into the Roman liturgy during the period in which Byzantine emperors appointed Greeks as bishops of Rome. The Gloria was probably also not sung on most Sundays in the Roman liturgy - at one point, it was only sung on Christmas. So the pope would process in with the ministers, incense the altar, and then immediately invite the congregation to prayer - oremus. What followed was the singing of a Biblical excerpt and a Psalm called the gradual, so named because it was sung from the steps, the gradus. Afterwards came the singing of the Gospel, and then the congregation offered gifts including the bread and wine which would be consecrated. After praying over the gifts, the ministers and most laity received Communion, while deacons held a long linen underneath their mouths, an instrument which isn't utilized today.

Let's talk about where things might not line up, though. For one, the Roman liturgy was stational, which means that the pope would visit the different Roman churches to celebrate Mass throughout the year. He would also send a piece of the consecrated Eucharist, called the fermentum, to the other Roman churches. I can't think of any instances where this is practiced today. In fact, Eucharist piety as a whole was different in these years - Eucharistic adoration and Eucharistic processions were completely absent, in contrast with contemporary Catholic piety.

Liturgically, the pope had a pretty strong presence in the city, but of course not every Christian in Rome, nor in the Christian world writ large, was attending Mass with the pope. We've got less evidence about how the priests of Rome, or the priests outside the diocese of Rome, celebrated the liturgy. It's also important to note that not every liturgy in the Latin Christian world was under the jurisdiction of the pope - the Arians had their own liturgical praxis, for instance, though I'm not really qualified to speak on it. Regardless, there are some liturgical experiences that today's Catholics wouldn't find in the late antique/early medieval Church. For one, we expect that at every Mass there will be a homily. This wasn't always the case. In fact, it was the Carolingians who insisted on vernacular preaching in the middle of Mass. I think it was also the Carolingians who insisted on the singing of the Creed - earlier Christians didn't recall the Nicene Creed during the Mass. This seems to be relevant to your question since the Church councils you mention probably exert their most explicit influence on the liturgy in this issue specifically. Every time Catholics recite the Creed in Mass, they're recalling the conciliar controversies of the past, but the Christians who lived in the centuries immediately following the Nicene Council weren't liturgically commemorating the Creed. However, I do believe that the Apostles' Creed was used as part of the Lenten reception of catachumens.

The catechesis of incoming Christians during this period is another issue. The liturgical texts still presume the reception of adult converts, but this is a period during which there's a shift towards infant baptism, confirmation, and communication (giving the Eucharist). Christians in this period didn't wait until reaching the age of seven to receive Communion, nor some other arbitrary age to be confirmed - all three sacraments were administered at once. Catechesis consisted of teaching the catachumen the Gospels, the commandments, the Pater Noster, and other Christian touchstones, as well as performing pre-baptismal exorcisms. These are some themes in work being done on the Gelasian Sacramentary and Ordo Romanus XI (though these might fall a bit outside of your timeframe).

This is already getting pretty long, and I feel like I haven't even scratched the surface of an answer, but here are the big takeaways:

I. The liturgical experience of lay Christians in the Roman sphere was defined by the figure of the Roman pontiff, and here contemporary Catholics might find some things similar, and some different, to their own liturgical experiences.

II. Theologically, the late antique/early medieval Church was used to inform the liturgical reforms. However, the actual laity's experience of this theology isn't clear. It certainly wasn't coming from liturgical recitation of the Nicene Creed, mid-Mass sermons, or the reception of catachumens.

III. The question is difficult due to the academic culture surrounding this question, and the assumptions of the scholars who have defined the historiography of the period. We might hope that an upcoming generation of scholars might bridge the gap between the studies of the classical and later medieval Christian worlds.

Sources and other thoughts in response below

My answer below focuses primarily on the doctrines of the faith; if you meant to ask more towards the practices of the faith, e.g. the language of the mass, common prayers and positions thereof, and the like, let me know in a reply and I'll see if I can get another answer up that touches on that more directly.

Before I go any further, though, a quick disclaimer — while I'll try to keep the potentially contentious components to a minimum, this is obviously a topic that reaches to the foundations of many extant branches of Christianity, so e.g. a Baptist theologian would likely portray a markedly different (and less Catholic-proximate) image of how the "original Christians" would have worshipped.

Finally, it's important to note that, even within the range you gave, the exact point in time you want to consider is going to have a pretty big impact on the answer to your question. This is because, as you mentioned, the 300s-400s were a very important time for early Christianity: of the seven ecumenical councils recognized jointly by the Catholic, Eastern Orthodox, and Protestant (among other) churches, four took place between 325AD and 452AD ^[1] .

To some extent, at the beginning of the span, beliefs and practices would indeed have varies widely. An Arian from a Germanic tribe circa 300AD would hold beliefs rather different from a North African Donatist at the same time, to be sure! However, that's not to say that it was a total free-for-all: after all, if there hadn't been some commonality between Christian beliefs at the time, there's no way they (or a significant subset of they) would have been able to get together and hold all those councils, right? Perhaps the single most important commonality between just about every sect at this time would have been the Eucharist, an element which would instantly clue a modern Catholic in to the fact that they were at mass, regardless of other trappings ^[2] . Other (some perhaps more controversial) practices at the heart of the modern Catholic faith that date back to before this time include the divinity of Christ ^[3], Confession and forgiveness ^[4] , the communion and intercession of saints ^[5], the veneration of Mary ^[6], and the resurrection of the body ^[7] . And when I say at the heart of the faith, I really do mean it: the Apostle's Creed reads^[8] (in part):

I believe in the Holy Spirit,
the Holy Catholic Church,
the communion of Saints,
the forgiveness of sins,
the resurrection of the body,
and the life everlasting.

Beyond that, one could also go further and argue about the similarities to be found in the structure of the Church, the primacy of the Pope of Rome, Apostolic Tradition, and so on, but that'd both belabor the point and cleave rather more directly to a Catholic historiography, something which I do want to avoid in the interest of impartiality.

By the end of that span, the key elements of the faith would have become even more comfortable for a modern practicing Catholic. In the course of the first four Ecumenical Councils, a multitude of differences, dilemmas, and disputes were settled, usually involving at least someone being cast from the Church for refusing to align with the council's decision — not the least of these being the Arians, the Donatists, the Nestorians, and the Monophysites. Notably, the differences between the beliefs of the Chalcedonians ('parent' to today's Catholicism, Eastern Orthodoxy, and Mainline Protestantism) and the beliefs of those declaimed as heretics are likely rather more fine-grained than would be understood by even a church-going Catholic, much less someone outside the faith; for example, one of the most notable proclamations of the first (and most divisive, by some accounts) council was that God the Son (i.e. Jesus Christ) is homoousious with God the Father, meaning 'of one essence'. That isn't to say that these differences weren't important — thinking about it from a mathematical lens, even one false axiom puts the whole logical framework in jeopardy — but simply to note that for anyone who hasn't spent their lifetime studying scripture and theology, as had the priests and bishops who attended these councils, many of these details would likely be easy to miss if they were to have a conversation with their friendly local Arian priest after being sent back to 315 AD.

It's important to note that these councils are not regarded as mere 'historical documents' in the Catholic Church of today, but as essential components of the deposit of faith ^[9]. In that vein, the first four councils settled a few other key points of faith that have persisted (as have the decisions of all other Ecumenical Councils) to the beliefs of modern Catholicism ^[10]:

• The Nicene Creed, which contains most of the 'core' beliefs that had been uncontroversial up to that point (incl. the divinity, death, and resurrection of Christ, the possibility of salvation and the forthcoming of the Last Judgement, and the existence of the Holy Ghost)
• The date of Easter (Nicaea)
• The position of Mary as Mother of God (Theotokos, lit. God-Bearer)
• The doctrine of the Trinity, via the Chalcedonian Creed
• The Holy Orders - though some (e.g. followers of Presbyterianism) disagree whether the definition would have included all of deacons, bishops, and priests.

Since then, at least in the Catholic view, nothing fundamental has changed in Catholic doctrine: things have been clarified and cemented, but the fundamental Deposit of Faith has not been altered. Such 'cemented' doctrines include the Immaculate Conception of Mary and that of Papal Primacy, both of which the Church supported via references to sources from the 1st-4th centuries A.D.; as such, while they would not have been doctrine during the timespan you mentioned, they would likely not have been considered entirely irregular, either.

So with all that in mind, I think it's safe to say that a modern, run-of-the-mill, church-going Catholic would little to dispute with priests anywhere from the ~350s onwards, at least not on theology!

1. https://www.oca.org/orthodoxy/the-orthodox-faith/doctrine-scripture/sources-of-christian-doctrine/the-councils
2. Various sources; I'd call notice in particular to Saint Ignatius, Letter to the Smyrnaeans 6:2, 7:1 (c. 110 A.D.)
3. Quite a few sources for this one, unsurprisingly — it was also cemented (rather uncontroversially) as doctrine in the First Council of Nicaea in 325.
4. Letter of Barnabas 19 (c. 74 A.D.); Didache 4:14 (c. 70 A.D.)
5. Methodius, Oration on Simeon and Anna (c. 305 A.D.)
6. Origen was the first to ascribe to Mary the title Theotokos in the early 200s, a title which later became doctrine with the Third Council of Ephesus (c. 431)
7. The Nicaean Creed
8. The Apostle's Creed
9. Good example of how they come up in the faith today
10. Wikipedia has a pretty good reference here if you want to poke around more

The answers you've gotten so far have been about liturgy and theology. I'd like to talk more about the practical side of the Church, how the Church was organised and how much that ecclesiastical structure has changed since the 4th and 5th centuries. For the sake of brevity, I'm going to be using "the Church" to talk about the Catholic Church, both then and now. This does not include the Orthodox Churches, the Oriental Orthodox Churches, etc.

Monasticism

First, let's talk about monasticism. In many parts of the world today, orders like the Franciscans and the Jesuits are omnipresent. There are a great many monastic orders in the Church today. Some others you may have heard of are the Benedictines, the Dominicans, the Cistercians, the Trappists, the Poor Clares etc. All of these, however, were founded after the time period you've asked about. Monasticism certainly existed in Christianity in the 4th and 5th centuries. Some early monastic saints from this period include Martin of Tours and John Cassian. These early monastic founders were influenced by the rise of monasticism in the East, led by figures such as Basil the Great, and they did found communities of monks and nuns. The earliest monasteries in some countries date to the end of the period you're asking about, such as the 5th century monastery at the Isle of May which is one of the earliest in Scotland. The most important of these was probably Lérins in Gaul, a 5th century monastery at which many future bishops were trained with a particularly monastic style.

However, formal orders, where monks and nuns followed a specific rule and were united by an international organizational structure, did not yet exist. John Cassian did publish his Institutes which were guidelines for his followers to live by, but you don't really have monasticism on a massive institutional scale until the Benedictines. And even as the Benedictine Rule gradually became the monastic gold standard throughout the Middle Ages, you don't really see enormous bureaucratic monastic institutions until the Cistercians and their ilk of the 11th and 12th centuries. Mendicant friars, who were much more present in urban communities with their itinerant preaching and begging, didn't really take off until the 13th century. And the Jesuits, one of the most famous monastic orders worldwide due to their aggressive missionary activities in the colonial period, weren't founded until the 16th century.

So your average 4th or 5th century Christian in the "Catholic" west would have maybe heard of monasticism; if they were learned, they certainly would have heard of it, but they may never have met a monk or a nun themselves. Monastic communities were few and far between in the 4th and 5th centuries. They mostly existed in remote locations far away from cities, since the point was to cast oneself into the wilderness to fight demons (literally). In fact, part of what made the Rule of St. Benedict so successful was that unlike some of these earlier monastic communities, which were designed with harsh desert conditions in mind, Benedict's rule was relatively less ascetic and could be adapted to different climates. This flexibility led to its spread across western Europe, although it wasn't until a few centuries after Benedict's time that anything resembling a "religious order" began to form around his monastic rule.

Women

Next, let's talk about women. Women in the 4th and 5th centuries had a much different role in the formal hierarchy of the Catholic Church than they do today. One of the thorniest things about talking about women's role in the early Church is the issue of ordination. The meaning of the word "ordination" went through a massive change in the 11th and 12th centuries. The word originally referred to anyone being chosen to play a specific sacred role in their community. While priests and bishops fell under this umbrella, like they do today, so too did monks, nuns, readers, widows, deacons, deaconesses, kings, queens, and other positions which would be considered lay rather than clerical today. A woman being ordained as a deaconess was considered no less ordained than a man being ordained as a bishop - they were just ordained to different roles. This changed in the 12th century with the clericalization of the Church, a period during which many tasks which had previously been available to other ordained people were restricted to priests (e.g. abbesses used to be able to hear their nuns' confessions and baptise local lay people).

With this in mind, women served several different ordained roles in the Church of the 4th and 5th centuries. Women were ordained as episcopae and presbyterae in some places during this period. These are the feminine forms of the names for a bishop and a priest. We have very few examples of women ordained as episcopae, so this may have been a relatively rare role. One example is an Umbrian inscription dating between the 4th and 6th centuries inscribeed to the "venerable woman, episcopa Q", who was possibly the wife of the 4th century Pope Siricius.

Five inscriptions survive in the West that are about presbyterae, all dating to between the 4th through 6th centuries. The most interesting of these is an inscription near Poitiers that commemorates how "Martia the presbytera made the offering together with Olybrius and Nepos". This is a typical reference to the Eucharist, meaning that a female priest called Martia celebrated the Eucharist along with two men. The evidence we have indicates that women did perform the Eucharist as ordained priests, but this was a matter of hot debate in the period you've asked about and beyond. The Council of Nîmes in 394 ordered that the ordinations of women as priests should be reversed and should not be done in the future. A century later in 494, Pope Gelasius warned the bishops of southern Italy and Sicily to stop ordaining women to serve as "minister at the sacred altars and to perform all matters imputed only to the service of the male sex and for which women are not competent".

These prohibitions continue, ad nauseam, throughout the early medieval period. What this tells us is that while there were always Church authorities who condemned the ordination of women, there were also bishops who had no problem ordaining these women anyway. The priests of Brittany were scolded in the 6th century for allowing women to administer the Blood of Christ - these women were derided as mulierculae, "silly little women", by the angry bishops of Gaul. And yet, in the late 6th century we hear again of a woman named Sirica who became an abbess but refused to stop wearing the vestments of presbyterae, instead of switching to the clothes expected of nuns. In the 8th century, Frankish authorities complain to the pope about how "women have presumed to serve at the sacred altars, and that the female sex, to whom it does not belong, perform all the things that are assigned exclusively to men". The repetition of the bans on women serving at the altar and being ordained as priests suggests that it was a persistent "problem" in the eyes of some Church authorities, and that the practice thrived across centuries in spite of calls against it.

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