How closely related is Religion and marriage?

Were those concepts invented by Christianity, or by individuals who happened to be Christian? It’s a lot bigger distinction than you seem to be realizing. By comparison, did Judaism come up with the Theory of Relativity?

I think the point though is to consider ‘belief in a God’ (or even ‘belief in the supernatural or spiritual realm’) to be absolutely central to religion, rather than (say) ‘acceptance of the importance of ritual’ or ‘acceptance of a particular cultural code of ethics’.

This is a position that is deeply informed by Christianity, which (in many cases) places “belief” right at the center of what it means to be religious - particularly the Protestant varieties.

Hence, the tendency to consider “atheism” the antithesis of “religious”, so a person would not, normally, consider themselves “atheist” and a follower of a particular religion at one and the same time.

This isn’t the case for all religions though - in many forms of Judaism, for example, it is perfectly acceptable to describe oneself as an “atheist Jew”, in a religious and not simply an ethnic sense–the reason being, that in Judaism belief in a deity/the supernatural isn’t as significant to the religion as ritual, ethics and lifestyle. There are plenty of “Jewish atheists”, whereas being a “Christian atheist” is more of a contradiction in terms - I man, they may exist (an atheist may be drawn to Christian ethics), but they would be more unusual.

My wife is Christian and finds this hard to understand, because to her, belief in a god (or at least some sort of spiritual realm) is simply what religion is. I suspect a lot of atheists who grew up within Christian culture feel the same way.

Well again I’m not so sure. I guess if my upbringing was anything it was Church of England but it is so wishy-washy that gods are pretty much optional. Certainly plenty of people go through the motions of religious ceremonies without thinking that belief in a god is necessary. I’d imagine that Atheist CofE people in the UK are probably very common.

Heck, Even my wife and I got married in a church and baptised our kids without ever in my life believing in a god. In my conversations with the vicars before those ceremonies I noted that the subject of me actually believing in god, very carefully, never came up.
So it may be that I’m already comfortable with the concept of Religion/Theism being separate.

True, there are loads of atheists who go to Christian churches - but they still tend to associate ‘depth of commitment to the religion’ and ‘seriousness of the religion in the first place’ with ‘belief in God’. As you note, even atheists brought up in a Christian cultural sphere consider churches where belief in God is optional as “wishy-washy”.

There are some things that are like Judaism within the Christian cultural sphere - the Quakers spring to mind, there are definitely fervently committed nontheistic Quakers, and there are other similar groups as well - but they are unusual. A combination of relatively strong personal identification with a religion and a lack of belief in any supernatural element is much more common within Judaism (There are plenty of fervent Reconstructionist Jews no-one would describe as “wishy-washy”, for example … ).

This gets to the heart of the distinction I made between ‘universal’ religions (Christianity, for example, which explicitly aim to convert as many people as possible, cross-culturally, and therefore blend with the local culture to the point belief in a dogma is the only trait all members of that religion have in common) and ‘cultural’ religions (Judaism, for example, which have no or very little evangelical drive, are wedded to one or a few cultures (Ashkenazi versus Sephardic versus Mizrahi… all specific Jewish cultures, all with some commonalities but all distinct from each other like they’re distinct from the cultures they’re embedded in), and which therefore need much less faith or dogmatism of their followers because they have so many other ways to belong, most of them more important); the more educated Romans certainly didn’t think belief in the existence of the pantheon was the central point of religion, and stated quite openly that the main function was preservation of social order. These days, the notion that religion is about social order is seen as an attack on religion in Western society, because most Western religions are closer to the Christian ‘universal’ model than the Jewish or Roman ‘cultural’ model.

(And I didn’t think I’d be pointing up the similarities between Judaism and the Ancient Roman state religion today.)

Definitely an interesting parallel - I admit, I’ve never thought of those particular similarities! But it does make sense - certainly Judaism is very much about preserving an orderly society, one centered on laws.

Judaism is a bit odd, because while it isn’t a universal religion, its system of laws has a “universalist” component to it - so much so that it actually (and unusually) contains a system of “basic universal laws” supposedly binding on all peoples at all times (the “Noahide Laws”) - with the notion that those non-Jews who conform to these Noahide Laws are, in fact, just as righteous as an observant Jew who keeps to specifically Jewish laws.

(In some ways, the “Noahide Laws” are similar to modern-day notions of universal human rights).

The point being: Jews saw no reason to “convert” non-Jews into Jewish beliefs, and had no intention of imposing specifically Jewish laws on non-Jews (they were for Jews only), but they did propose to judge the morality of non-Jews by holding them to the “Noahide” standards.

You underestimate the diversity in the US. In Pennsylvania, where I got married, you do not even need an officiant to get married, due to their Quaker heritage which did not include ministers etc. My daughter and her German husband got married in the US partially because in Germany you either have a religious ceremony or a very restricted civil one. There are no such restrictions in the US. I got married in an atheist institution (nice hall) and neither of my daughters got married in any place remotely religious. I’m not sure any of their friends had church weddings per se.
The City Hall in San Francisco is an extremely popular place to get married.
I’m sure there are regions of the country where church weddings are prevalent, but not around me.