Hey. Something I can answer.

So since 1970 or so, there has been a new rite published by Pope Paul VI which is known as the “ordinary” form today. It’s the Mass from after Vatican II, a big ecumenical Council, that’s designed to adhere to the goals of Vatican II: transparency by understanding the language, switching from Latin to the vernacular, more active participation by the laity, and a focus on the laity as the “People of God.”

It’s the Mass you know: in the vernacular, table altars, versus populum (priest faces the people). (Due to all this, there are people on the Internet who claim that this is a new religion due to all the changes, the pope isn’t legitimate, and the whole modern church are apostate. Those are sedevacantists, or those who believe the Chair of St Peter is vacant.)

Nevertheless, the general flow and order are mostly the same. Process in, prayers, the Penitential rites, Kyrie, Gloria, Collect, Readings, Alleluia, Gospel, homily, Creed, Preparation, Canon, Our Father, Agnus Dei, Communion, Post-Communion, Recessional. There are a few things missing from the traveler’s time but still overall recognized.

Now also, there is a Mass called the “Extraordinary” Form. It is the Mass from and before Vatican II. The Missal (book of prayers) was last updated in 1962 by John XXIII. It’s in Latin, the priest faces the altar (called ad orientem or facing the east, where Christ was believed to rise from), altar rails, and a focus on the liturgy and not the people. This Mass follows the same structure I mentioned above, and it had not changed since at least 1570. That’s when Pope Pius V released Quo Primum, the document making this rite the only one the Church will use (there are other rites he allowed, to be done, specifically ones at least 200 years old, but that’s reaching out into technicalities) So that rite hasn’t really changed since 1570.

There were some major changes specifically to Holy Week in 1955, such as removing readings, rearranging times, cutting down on repetition. See Pius XII’s Maxima Redemptionis for specifics, which I will summarize later.

Before that? Well, there’s an issue. I have to go to work and will return to continue the talk about prior to 1570. But since I wrote a thesis on liturgical changes and Vatican II, I’ll look at that again and get more prior to 1570 for you.

TL; DR: There’s enough basic flow in the Ordinary Form that arose in the 1970 that past people would get what was going on but they would be more at home with the Extraordinary Form, which is the same basic Mass they would have gone to since 1570...apart from the Holy Week ceremonies prior to 1955.

Edit: See below for Part 2.

I would like to add to this question. What about for the Orthodox Church, we're told that the church hasn't changed liturgy since the 300s, is this true? Can this be proven?

Well, you are sort of asking two different questions here considering the wording you chose. There's also a certain difference in age involved when talking about this particular topic.

• A catholic mass would be recognisable to a practising catholic back almost certainly until the 16th century when the council of Trient happened which was more or less the second last big reform in Catholic liturgy and maybe until the 11th century when the church first introduced a unified liturgic system. Mind you, he could most likely not participate in a mass from that time period but whilst many things changed in the way mass is conducted, some have not which is why I believe it would still be recognisable.

• The second thing you mentioned was if a modern catholic would be able to paricipate in a historic mass and the answer to that is that it depends on your age. The second vatican council ended in 1963 which was not all that long ago. This means that quite a few people are still alive that were raised with the pre-council version of mass. Considering that my parents to this day know the latin words for things like the pater noster means people of that generation could most likely participate in mass to a certain degree. Also note that catholoc mass back then was way more centered around the altar and the priest himself than it is today.

If you are familiar with a modern Catholic mass, this web site, "Experience of Worship" will give you an idea of the similarities between a medieval and modern mass. It recreates what a late medieval mass in England would be like. Other than the Latin language and the relative passive congregation, most of it is very recognizable.