I have a theory.
Religion, near as I can tell, is about social control. Not necessarily by some Pope over the laymen, but control by a society of the society. Remember that, in the beginning, it was extremely different. You didn't have government in the usual sense; you didn't have laws or such. But these things grew quickly.
In a human society, you need two things: you need rules, like "when the leaves start to fall, it's harvest time" or "no one crosses that bridge" (that way lies dragons!); and you need explanations (because human beings are curious by nature).
Rules, over time, become laws. What might once have been, "it's hard for people to work every single day", becomes "this day is reserved for not working".
Explanations evolve into stories of culture heroes, where persons of mythical to godlike status go on a journey and do some amazing feat, which explains why such is so. An example is Noah and the Ark, which is also seen in Iroquois mythology as The Great Flood, and in Babylonian as Gilgamesh. (
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When these are balanced, you achieve what I would call a Mythology, a belief system in which stories are as important as laws. Based on what little I know of Catholicism, this is one. A belief system that favors stories over laws I would call Folk Tales, whereas one that favors laws over stories I would call a Religion. Protestantism is, essentially, a religion.
If you'll notice, most people who interpret the Bible will disregard all literal depictions. Everything is merely fodder to derive laws from; people love stories, so it's unsurprising that some of it survived, but I think the Christian stories survive more because of a persistent Catholic influence. Apologists have been taking cues from them.
So, what is the purpose of a religion? To justify law. What is law? A set of social agreements about behavior, including penalties for breaking these agreements. For instance, everyone agrees that they won't "covet their neighbor's wife". If you do, everyone also agrees that rocks will be thrown at your head until it's a messy red stain on the ground.
And why do people get defensive about it? Because it's theirs. Sometimes, the laws a religion includes into its tradition becomes outdated. But they're part of the religion whether you like it or not: people have a hard time taking it piecemeal, because that requires a lot of analysis and consideration. It's easier to say, "It's all true," especially when saying, "This part is true, but this part is false," gets very messy very quickly. And who wants to deal with that? Much easier to stick up your nose and call them an infidel, heretic, blasphemer, whatever.
And the fact is... we don't need religion anymore. With a secular governmental system in place, religion as a set of laws becomes unnecessary. We still need folk tales and mythologies, though, because stories are important. And the legal system we have today... it's not so good, still. But it's better than divine fiat, where the legal authority is a little hard to have a chat with.