I've been looking into marriage traditions and found some sources that claimed that in the early days of Christianity, marriage had nothing to do with the Church and was simply a legal agreement. From Garry Wills:
The early church had no specific rite for marriage. This was left up to the secular authorities of the Roman Empire, since marriage is a legal concern for the legitimacy of heirs. When the Empire became Christian under Constantine, Christian emperors continued the imperial control of marriage, as the Code of Justinian makes clear. When the Empire faltered in the West, church courts took up the role of legal adjudicator of valid marriages. But there was still no special religious meaning to the institution. As the best scholar of sacramental history, Joseph Martos, puts it: “Before the eleventh century there was no such thing as a Christian wedding ceremony in the Latin church, and throughout the Middle Ages there was no single church ritual for solemnizing marriage between Christians.
After the fall of Rome but before the eleventh century, how did people get married? Was the wedding officiated by a legal authority? Was there simply no officiant, and if so, was there a ceremony/celebration at all? Legitimacy of heirs seems very important to the nobility but less so to your average peasant, so peasants still get married?
I'm particularly interested in the traditions in France and Britain, since that's where I'm from, but anything ruled by the Catholic Church still counts.
I'd disagree with the interpretation given by Gary Wills on this as well as many other things in that its really a stretch to argue the state of marriage in that paragraph as to there being "no religious" meaning to the Early Church. That's hardly the case and marriage was in fact addressed in various ways from the very beginning of Christianity. To the extent he has a point, it's that over time the Church (and in the West and the East) began to officiate over weddings during the Middle Ages (at least in the West) and at least in the West the theology of sacramental marriages was developed in that period. The Orthodox have a theology of marriages being sacramental but it differs from the West and I'm not sure when it developed, although they cite to the earliest eras for that view.
One thing that is not really grasped by many even now when they look at this topic that marriage, from a Catholic prospective, is contracted between the parties to the marriage. That is, while a Priest now officiates as the wedding, it is the parties to the marriage themselves that contract it. The parties marry each other, the Church doesn't make the marriage. Having a Priest present at the marriage became a matter of Church law during some point during the Middle Ages, but not because it was thought that having a Priest present was necessary in order to cause the marriage to come into being but rather because the lack of witnesses, etc., caused obvious problems to potentially exist. Catholic marriages are regarded as sacramental, or can be so regarded, which adds an element to this that I don't know the answer to, however. Contrary to some common myths on this, the Church (East and West) recognizes "natural marriages" even now when contracted by those who are not in the Catholic or Orthodox churches. Now marriages not officiated over by a priest are regarded as invalid as they break Canon Law, not because a Priest is required in all cases to make any marriage valid.
Anyhow, prior to church officiated weddings being common the parties simply determined that they would enter into the marital state. That's pretty simple, but that's what occurred. Publicly acting as married and entering into the incidents of marriage, such as common household, were sufficient for a marriage to be recognized, somewhat like Common Law marriages today where they are recognized. Peasantry were indeed married. The period before a wedding might be long or short and actually entering into a marriage was largely without ceremony. But then, if we keep in mind the level of ceremony we see today, even in non Church weddings, we should also keep in mind that this is somewhat of a recent development to some extent and it wasn't all that long ago that a lot of weddings were very simple.
Marriages of this type remained common even into the 11th Century in some places. For example, I believe that King Harold Godwinson had such a marriage to Edith the Fair and while it is sometimes claimed that this caused that marriage to be regarded as illegitimate that isn't really terribly clear. Certainly some monarchs prior to that time had marriages that were officiated by Priests but not all did.
It was not until 1184 that the Church determined to cause Canon Law to require a Priest to officiate at a marriage, so that was all the way into the late 12th Century. It wasn't until that time that a formal ceremony was developed by the Church in order to officiate at a marriage. As noted, prior to that it was licit for the parties to simply agree to be married and then they were regarded as married, and the Church recognized that. Following 1184 a Priest was required and a standard ceremony was performed. The Council of Trent created the further requirement of there being two witnesses to the marriage. It was performed on the steps or entry way of the Church (depending upon the Church), not within it, up until the 1600s.
It should be noted that the Orthodox understanding of the sacramental nature of a marriage is somewhat different in that it is not conveyed by the couple themselves but rather by the Holy Spirit at a ceremony requiring a Priest or Bishop.
One thing that is worth noting is that the requirement for witnesses and hence a more formal procedure is coincident with the slow rise of wealth, and hence inheritance, in society. When most people had nothing, their marriages were important to them and their immediate family, but not so much to society at large in terms of property. Serfs bound to the land did not own it, for example, and therefore there would never have been an inheritance dispute concerning what little they had. When they started to have something, however, that began to change and the need for a wider societal recognition also existed to address that situation.
Likewise, the modern situation we're so accustomed to of people obtaining a license to marry (or in some places in Europe engaging in a civil wedding followed by a church wedding) is a byproduct of the Reformation when it became necessary for the state to keep track of what faiths people were marrying in. Had, for example, the English Crown not imposed a license requirement upon England there would have been no way to know who was marrying in the illegal Catholic faith as opposed to the official state religion. In later eras when various governments became anti clerical that requirement served the same function. Now, we're so used to it that we can't imagine it really not existing, although there's at least one American state that apparently doesn't fully require a license in order for a valid church wedding to be conducted in their state.
If you’re interested in France and Britain, I would highly recommend Law, Marriage, and Society in the Later Middle Ages by Charles Donahue, jr. He goes through an extensive number of English and French marriage cases.
The requirement for a priest to be present in order for the marriage to be valid only starts in the Catholic Church with a decree of the Council of Trent, Tametsi. Prior to this, the presence of a priest, witnesses, banns were required for a licit marriage, but not a valid one. An illicit marriage might result in some sort of ecclesiastical censure, but the marital bond itself would still stand.
There is a great deal of debate among twelfth-century canonists and theologians over the requirements for a valid marriage, but the issue is mostly settled by the late twelfth century. Pope Alexander III established the basic contours of marital formation that would stand until Trent. Without getting lost in some of the details, the main constitutive element of a marriage is the consent of the parties. Words of present consent exchanged between of-age men and women, or words of future consent followed by intercourse would constitute a valid and binding marriage. Informal, clandestine marriages figure prominently in marital litigation. Just as an example, Donahue notes multiple instances of a spouse seeking to annul a marriage to an existing spouse on the grounds that he/she had already contracted a clandestine marriage with someone else.
If you want to read up on this all from the legal side, in addition to Donahue’s book, I’d recommend James Brudage’s Law, Sex, and Christian Society in Medieval Europe. If you can access it, I’d also recommend Cordelia Beattie’s article “Living as a single person: marital status, performance, and law in Late Medieval England” Women’s History Review 17.3 327-340.