[I]f you ask about 99% of Christians where this whole idea of suicide being a sin and a bad thing came from, they will say, "The Bible," "Jesus," or, "I don't know." The correct answer, of course, is that it comes from St. Augustine's City of God. There's a bit about suicide in the Old Testament, but at times it is said to be wrong and at other times it is pretty clearly seen to be a heroic act. But never anything like an "unforgivable" or "mortal" sin as many Christians believe today. And Jesus says not one single word about suicide, nor does Paul.
[E]ssentially, Augustine argued that suicide was a sin in the eyes of Jesus more or less because having Christians get eaten by lions was good for recruiting. Which really doesn't apply anymore. And, in any case, almost no Christians know where the 'no suicide' dictum even came from. As such, why do Christians all seem to know it and cling to it so fiercely? Because, in the dark of night, when they're all alone and scared, in their deepest heart of hearts, they doubt that God exists.
Augustine's dictum is a way of dodging the truth: that if they were really, really sure paradise was around the corner, they'd be a lot more anxious to go there. They wouldn't have to commit suicide. Just take a dangerous, but good job, and get killed helping others. Not only is that good deed your ticket to Jesus' buffet table in the sky, but it's a quick ticket too. But they don't do that. Instead, Christians stay here on Earth and spend a hell of a lot of time worrying about what other people believe and trying to define atheists out of existence.
This author discusses an important issue which I have only touched upon here and there before: believers' attempts to appropriate the power of organizations like the state may largely be the product of a need to reinforce their own beliefs against the power of doubt and skepticism. So long as the state doesn't back their religion, then alternatives are presumed to be just as good. This isn't acceptable to someone who doubts yet doesn't want to disbelieve. People who are weak in their faith need regular reinforcement and propping up — and what better means for achieving that than the coercive power of the government?