I'm a Mormon. (I hope you've realized this by now, take a look at my sig.)
I believe everyone is judged by what they know, so people who never hear of the "true" gospel can still go heaven and the celestial kingdom.Oh wait, am I the only one who knows what the celestial kingdom is?

So even if not everyone knows about the religion, its still good to tell people the truth, right? And get more people to follow the good standards. Even if they don't believe, its still good to do what right. And along with that, for the second coming, every nation has to hear of the true gospel or something like that. And our church does baptisms for the dead and things like that.
Almost everyone in a religion thinks their religion is right, or they wouldn't be in it.(atleast the ones that actually go to church) I'm one of them...