Gary Wills dispels the contemporary fundamentalist Christian myth that marriage has always in Western culture been primarily a religious institution:
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.”
Only in the twelfth century was a claim made for some supernatural favor (grace) bestowed on marriage as a sacrament. By the next century marriage had been added to the biblically sacred number of seven sacraments.
And vorjack adds:
The church fathers ranged from men who thought that marriage was a lesser good than celibacy (St. Augustine) and those who thought it a lesser evil than fornication (St. Jerome). Most seemed to agree with St. Paul that “It is well for a man not to touch a woman.” (1.Cor 7:1)
The Church came to marriage late and grudgingly. Only in the twelfth century did Aquinas add an Aristotelian spin on marriage and make it a sacrament. Note that this is not a biblical argument but a natural law argument. Protestant founders like Luther and Calvin seemed to reject it when they left marriage as a civil institution.
vorjack references these considerations to rightly push back against the dubious suggestion of both fundamentalist Christians and libertarians alike that the government end civil marriage as an institution altogether. vorjack creatively suggests that Christians just rally around the word “covenant” if they want a word for their unions that is distinctly religious (at least in modern time) and not equatable with civil marriage.