There would be many reasons to believe that the Abrahamic God doesn't exist or if he does, has been obscenely misrepresented though writings that resemble the knowledge base and morality of ancient Mesopotamia more than an omniscient, otherworldly, divine, loving entity. Those writings alone would be powerful evidence enough that if God exists, he doesn't want us to know very much about him.
The real kicker for me, however is just the general way so many Christians behave. It would be easy to point to a wave of Pedophiles Priests or Southern Baptist clergy, but I get all people make terrible mistakes and do terrible things. I also understand people might use religion to coax money out of other the more vulnerable. Many, many of us seem to be predisposed to prey on others-as a Republican Christian joked to me one time, "I wish we lived in a 'praying' society instead of a 'preying society. What I don't get is the ridicule and loathing that I see on a daily basis from those that should be at the very forefront of spreading love and kindness. Look no further than this board for a brief example. Forget about comparing Christians to Atheists-I don't know that Christians treat others better than any random group of Gorillas.
I also get the argument that the Church is a Hospital for sick souls. Fair enough-I'll buy it. Seems to me, however when one is in the Hospital, one wants to get OUT of the Hospital. one does not assume the Hospital is en excuse for bring permanently sick. If I'm in the hospital, I want to get out of the hospital, and if I need to change my behavior, so be it. Christianity seems to imply, if not outright say, "well we're all filthy sinners who are damned before God anyway...no need to not call my liberal brother a liar or a hypocrite just because he disagrees with me.