I posted to this a long message to this thread, yesterday, during one of the brief times that Mudcat was working... and the 'Cat stopped working in the middle of the [Submit Message] procedure... I was hoping the message squeaked through under the wire, but it didn't. So I'll try to sum up in two (or more) lines:

1) Don't just write (or sing) what you know...

So many people seem to think that the old saw "Write what you know" means they have to be more or less autobiographical. But the truth is, we know much more about life than the stuff that statistics (such as beliefs about religion) reveal about us. The important stuff, such as our emotional compasses, tends to be stuff we all share. 2) ...but what you Believe

As others have said before me, you have to have some integrity, some conviction, behind what you are singing. So -- to take the song you gave as an example -- you don't believe in the song's message about Jesus of Nasereth. But there probably is something in that song you can believe in, even if it's not in the surface message (maybe the idea conveyed not to give up hope), or you wouldn't have called it "a really cool song".

To use your second example... I am a tea-totaller, but I love singing drinking songs. Most of them are not "about" drinking, anyway, but about friendship and living with enthusiasm -- and those things, I can champion with a clear conscience, because I agree with them.