In my studies of religion, I keep running across a very common fallacy, the appeal to consequences. It is very widespread. Basically, this fallacy is arguing that some proposition is either true or false based on the consequences of that proposition. For example, one might argue that without a belief in God, one cannot have a belief in absolute moral values, and therefore God must exist to allow us to believe in absolute morality. Another example would be that evolution leaves no room for God, therefore it must be wrong. It is usually an emotion driven argument, primarily propped up by fear or anger, although sometimes it is simply a lack of understanding or imagination. In the case of the "existence of God" example, it was driven by a fear of having one's moral grounding undermined. In the second case, the person was simply unwilling or unable to imagine the possibility that there could be some form of theistic evolution.
I bring it up, because I just got done listening to a partial sermon by the Christian apologist, Ravi Zacharias. His entire message was based on this fallacy(although his further sub-arguments revealed a few more fallacies): I can not be an atheist because I can not live with the consequences.
Of course, such a statement has no bearing on whether Ravi's god actually exists. One way to point this out to someone using such an argument is to turn it around: I cannot be a Christian because I can not live with a god who would create Hell, for example. You can then point out that your feelings about a god who would act in such a way have no bearing on whether that god exists, a point which they would certainly agree with as a theist. But, be ready to deal with an emotional blockade whenever this argument rears up, because in my experience, it is almost always driven by pure emotion.
PS - I did find it funny that Ravi took the time to point out his respect for agnostics who have taken the time study the issue of the existence of god seriously. He is a little mixed up as to the difference between atheism and agnosticism. One is a question of knowledge(can we know that god exists?) and one is a question of belief(do I hold to a belief in god?). So, he really spent the time lauding every atheist who is not just a knee jerk non-believer. Thank you, I guess.
EDIT: I just noticed the Wiki link for this fallacy:The Appeal to Consequences