I became an atheist about 10 years ago when I was 19. It was a fairly gradual process, I was basically a deist by the time I quit religion altogether.
Very basically, the clincher for me was when I got to the point of no longer being able to believe in God because if God did exist I thought that he must be evil. Why would God create people with the sole purpose of worshipping him to get into this heaven? God appeared to be a meglomaniac like Hitler or Stalin.
In fact, Lucifer (who I didn’t actually believe in) appeared more good than God by my evaluation, because Lucifer had the balls to stand up to the totalitarian God. There were many pertubations and illogical problems based on the ethical contradictions.
Interestingly for me, my girlfriend became an atheist primarily for problems she had with the metaphysical qualities attributed to God – ie: the cosmological arguments which Diana at NoodleFood has covered so well in her current podcasts on “Rationally Selfish Radio” http://www.rationallyselfish.com/blog/
Anyway several years later I came accross the philosopher Epicurus and the first formulation of “the problem of evil”. I often think of the problem of evil because it described the types of thoughts I was having at age 18-19. However, whenever I try to quote Epicurus during discussions, I usually forget the exact words, so I’m putting them on my blog in an attempt to memorise the quote:
“Is God willing to prevent evil, but not able? Then he is not omnipotent.
Is he able, but not willing? Then he is malevolent.
Is he both able and willing? Then whence cometh evil?
Is he neither able nor willing? Then why call him God?”
Philosophers would use this type of ethics based argument to suggest that evil itself doesn’t exist or to tackle the problem of defining evil. But for me, this type of argument was most powerful as a simple examination of the logical implications of attributing certain qualities (ie: perfection and perfect goodness) to the vaguely defined, concept “God”.