This is actually an idea I've discussed with others. And we came up with a pretty interesting, maybe far fetched idea of how that might be solved. Those who have other beliefs will pass away and show up at the gates of Heaven for judgement. If they have lived innocent lives as well as helped the needy, avoided doing negative or adulterous things, and are worthy of another chance, then they will meet Jesus and he will say something along the lines of "Hello my child. This may be all a but shocking or even confusing to you. But you have a choice. You can join me in heaven and live a perfect sinless life while finally learning the Christian faith with complete confidence of it's truth, or you can choose to deny me and serve the rest of eternity in Hell." We also came up with the idea of a sort of "realm" where those individuals go to in order to transition from their previous belief to the next. A very good question that I could even ask my own preacher.