If it wasn't common knowledge that Jesus died for our sins etc., and only a few select people knew and went around proselytizing then it would make sense, but literally 90% of the world's population knows about Christianity and other religions, and if some still choose to reject it then there's no reason to go around trying to convert everyone. If you're religious, that's okay. If you're atheist that's cool too, and if you're agnostic you're an indecisive douche. I mean, who isn't open to the belief that God exists? If an atheist was shown proof he'd no longer be an atheist, which makes being agnostic pointless.