I'm an agnostic-atheist. One of the biggest problems I see with theism is the inability to justify objective moral values and duties if you have a theistic worldview.
Let's take Ted Bundy. He raped and killed women and girls. A theist can believe what Ted did to be wrong, but a theistic worldview doesn't allow for the condemnation of Ted's actions. You may think (and feel) that Ted was wrong to rape and murder, but ultimately one's deity/deities don't care. Moreover, Ted doesn't agree with you: he thinks and feels that he's completely justified in murdering/raping. Ted just isn't interested in playing by the rules.
The question now becomes, whether we can call what Ted did 'wrong'. I see no reason to suggest we can (based on a theistic worldview). Can a jury rightfully sentence Ted? On what grounds/rules would a jury sentence Ted? We can't use "gods" moral rules because Ted never agreed to play along. This leaves us with his own rules, but his rules don't condemn his actions, they justify them. See, the problem isn't that theists can't be moral, the problem is that theists can't expect someone else to be. And a society that can't expect everyone to adhere to moral laws is a society I do not want to live in.
At some point in the distant past, the universe went through a phase of cosmic inflation, Stars formed, planets coalesced, and on at least one of them life took root and developed into the human race, who conquered fire, built societies and developed technology .
All of that so we can argue about nothing.