It's hard to say; it's like wondering when infatuation becomes love. I don't even know that there is a firm line between them. I think accquaintanceship merges into friendship as you get to know and like each other. Obviously some people move farther along and become closer friends than others.

A friend knows your flaws and likes/accepts you anyway, is one definition I like. Someone whose well-being is essential to my own.

But I have friends whose religious and/or political beliefs are vastly different from mine; sometimes that's ok, depending on what those beliefs are. I wouldn't be able to be friends with someone who was racist or sexist, for example. But I have some religious friends. It works because we emphasize the things we *do* have in common, although one of those friendships didn't work out because she became so religious that literally every part of her life became centered on her religion in some way (she even quit her job to become a missionary), so we had nothing in common any more.

But those friendships work because I accept that they believe a bunch of nonsensical superstition, and they accept that I resist the need to live my life as God requires.