Have you read the Chronicles of Narnia?
C. S. Lewis has a character that follows Tass (which is the other evil religion). Well, during the book The Last Battle, the followers of Aslan (Christ) go through this barn door to heaven. When they are in heaven they see this follower of Tass and ask how he got there. He says something like that he was always trying to do the right thing, and that he thought Tass was the correct god. However, when he got through the door he realized that he was really trying to follow Aslan all along.
Somehow after this character got to Heaven he got a choice to follow God.
I don't know if I explained it correctly. However, I think what C. S. Lewis is saying is that people who have never been exposed to Christ in this world will be given a final chance to follow Christ in Heaven. I also recall reading in the Bible that somehow everyone will be given a choice to follow God.