In my country legal and religious marriage are strongly related: you cannot get married in the church until you get married in front of the mayor. So it is not that obvious as you suggest it is.

You wrote, "then the states stole the term" as though religions own the idea. Religions shouldn't dictate law, and where they don't, whether you can get married in a church means nothing.

That's how it should be, unless you're cool with the idea of the possibility of some other religion becoming the law in your land.

I don't care about religion, but if the church invented the notion of marriage a thousand year ago why not let them use it and for state affairs use a distinct term? I would not consider the religion to be any sort of law in the land, just have a copyright on the "marriage" as a wine maker can have (ex: DOC). People who believe in a religion can have their marriage, people that don't practice a religion can have something called differently because it is differently. Religious marriage in my country has zero legal implications, it is just a ceremony in the church.

> if the church invented the notion of marriage

It didn't. Specially not the Christian church. The Christian bible was the basis for the christian religion. It was written before the religion existed. And it already mentions married people.

We have evidence of marriage going back to the 2300's BC. And it probably existed before that, we just didn't find the evidence yet.

> if the church invented the notion of marriage a thousand year ago

Well, first the notion of marriage is a tad older than a thousand years, to claim it's only that old is remarkably ahistorical. Second, which church? Many religious and non-religious institutions have had some notion of marriage for quite a while. Which one gets to claim first rights and to have the proper definition and why do they get to keep making that distinction today for people who aren't members of their church/religion?

Nah, we have very few writing from the Celts, but they did practice marriage and divorce (the woman took back her parents' bride price, and could divorce if her husband talked about their sex life. The husband could divorce if his wife insulted his beard). The also had concubinage contracts, probably with at least one-to-many, maybe many-to-many.

It was purely legal/civil matter, because religious matters weren't written down. So marriage was in fact not religious originally.

I've gotten that from Peter Berresford Ellis' books, which contains much more than that, and paint Celts as a more refined society than the greco-roman one.