If Christianity is true, then reaching the correct belief (about Jesus) is necessary to avoid damnation. Here is a brief video highlighting one of the more unpleasant implications of Christian theology. Admittedly it is not a logical argument, however since most believers tend to believe for emotional reasons, bringing up issues like this can be very effective at "putting cracks in the shell" of faith.
Here is the main problem: the assumption that some people deserve to go to heaven and others deserve to go to hell. People think that just because they haven't raped or murdered someone or broke into someone's own house and stole their property illegally that they are better than other people who have done things that they would not do themselves. They think that if there is a God their standard of morality is good enough to warrant their right to be in heaven. It's not. God's standard is not just moral..it is Holy. Not one of us measures up! This is why we we can only be in heaven through Christ. You might not be a serial killer....but we are all serial sinners. I agree with all the scriptures in the video. We are only saved through Christ. However I can't look at anyone's life and gauge with absolute certainty whether or not they have given their life in Christ or if they ever will become born-again. I don't know what the 15 year-old girl's relationship with Christ was. If she didn't have one, she didn't make it. Same with her killer. I deserve hell just as much as he does. If God saves him, why should I be angry with God? He takes sinners and makes them better - transforms them from evil doers to good doers.
One of the comments on the post really cracked me up!
So did the thief on the cross. If God chose to save them what is the problem? I think it's a bigger problem for you if they end up in heaven with you in hell - all because they humbled themselves and accepted Jesus but you rejected Jesus. That's a really stupid reason to miss heaven just because you don't like who might be there.
Debunking Christianity: Who goes to heaven? Who goes to hell?