According to Christianity, who can go to Heaven?

Honestly, I don't mean to be offensive or judgemental of anyone's religious beliefs. I am a Christian (although I don't really go to Church) but I've never really liked the idea that the 5 billion people who aren't Christian are going to hell. What about Gandhi? What about indians in the Amazon who have never met a missionary? Again, I am asking in all seriousness, and not to ridicule anyone's spiritual beliefs. Thanks.