Most people professing a religion have been born into it. That makes it less a matter of choice for them than of accident. For others it may be a question of compulsion, enforced by theocratic states.
For all of these, and also for more genuinely voluntary adherents, it’s also a tribal thing, giving them the comfort and security of belonging to a group, often – though not always – directed against other tribes; usually other faiths, as in the Crusades, but even against those supposedly professing the same religion: Protestants against Catholics, Sunni versus Shia, Orthodox against Progressive Jews, and so on. It’s hard for us agnostics to understand this – what on earth does it matter if the communion wine after the priest has done his mumbo-jumbo over it really is Christ’s blood or not? – but there it is. But that only makes it the more likely that issues like transubstantiation are not the ones at the root of ‘religious’ differences, but more common-sense practical and often political ones. (To be fair, I don’t imagine that even most of the religiosi would disagree.)
Religion can also be perverted, in the interest of politics, again. The most recent example is the way ‘Christian Nationalists’ in the USA have totally misunderstood what appears to me (I was brought up a Methodist) to be Jesus’s pretty clear teaching on matters of individual and social behaviour – love your neighbour, welcome the foreigner, turn the other cheek, swords into ploughshares, and so on – in order to harness it to some very un-Christian causes and people. I’m thinking here of course of those ‘MAGA’ Republicans who seem genuinely to believe that Donald Trump was sent by God to ‘save’ America, or to ‘make her great again’, or whatever; or even that he is Jesus Christ in His Second Coming – a famous image of which he must have liked because he reposted it himself, before being made aware of its blasphemy (https://www.bbc.com/news/articles/c17v8y0z9z2o). This has famously gotten him and the entire MAGA movement into deep trouble with the American-born ‘Woke Pope’ Leo XIV, the head of one of the main Christian churches; who some Trumpists are claiming doesn’t understand Christianity as well as they do.
Is this what is taught in the ‘Bible Belt’ these days? Tribal rather than more liberal Christianity? The Old Testament rather than the New? Is it significant that the bits of the Bible that Southern Christians want compulsorily plastered over all schoolrooms are the Ten Commandments (OT: Exodus 20), rather than the Eight Beatitudes (NT: Matthew 5:3-12)? In other words, are America’s famous ‘Judeo-Christian’ values more ‘Judeo’ than ‘Christian’? That might help explain her diplomatic and military closeness to Israel.