Regarding how long the secret can last- I have asked her that too, and she said that's a future problem, not right now, because "we're comfortable where we are now".

I have said something along those lines about the consideration of others versus me. She simply retorts with saying the same thing back- why should I care about other people knowing when the relationship is just me and her. She does treat me like her boyfriend when we're not around friends.

I'm 99.99% sure this isn't related to being unfaithful, for what I know, and that we've been best friends for long enough that I know her inside out.

In terms of religion, she is religious and has issues with telling her parents because of it. That's perfectly understandable for not wanting to telling her parents, but that doesn't excuse telling our closest friends. (We share the same friend group, and have the same close friends)