Religion rains on the just and unjust alike
From a philosophical standpoint, the main attraction of religion is that it can ostensibly give man an absolute, timeless, and universal standard of justice. Even for a moral realist like me, this is an attractive claim prima facie. It would be like offering the Theory of Everything to a physicist in a nice leather-bound book. The Christian's claim that we reject God because we shirk its judgment does not in fact apply at all : who wouldn't consider such an offer ?
There's just one little problem... it's complete bullshit.
First of all, religion does not, and cannot, provide an absolute moral standard. It is a fact of memetics that a belief system like Christianity must adapt or perish. As social norms change, Christianity must also change to preserve its moral validity in the eyes of the people. And we indeed observe such a relativism in history. Not only do we observe it, but the numerous schisms in Christian sects in itself is proof positive of that relativism. If there was a singular unchanging moral scheme, then why should more than one sect exist at all ? There would be one way of doing things, and that way would correspond to one organization.
I imagine a Christian may reply that the existence of all these sects is the result of human error. But it is part of a well-designed scheme that it is self-corrective, like scientific progress for instance. So why isn't religion self-corrective ? Why is it the exact opposite - constantly dividing ?
Secondly, and this is a familiar argument to most atheists, the brand of ultimate justice meted out by the Christian god is not justice at all. We are being asked to believe that eternal bliss and eternal torment (at least, that's how Christians traditionally interpret the very vague Biblical descriptions) represents justice. What crime can possibly justify eternal torment ? What good action can possibly justify eternal bliss ?
The Christian once again objects at this point that humans deserve eternal torment because they offend an all-powerful being. So justice, in the Christian worldview, means that one must be punished proportionally to the power of the "victim" ? Should killing a hobo be blameless and swearing at a president earn one an automatic execution ? Once again, this is not justice.
Thirdly, the fundamental doctrines of Christianity are the clearest examples of injustice you could ever find :
* The concept of "original sin", central to the need for Christianity, is an evil done by the first human which somehow applies to every human being.
* Salvation is based on the premise that some people, by virtue of belonging to the correct religion, are "saved" while everyone else is not. This salvation is then reified as the highest form of justice.
* Salvation came to the world thanks to "Jesus", who died for other people's sins. Somehow a man's execution (and why an execution ?) is supposed to redeem other people's sins, perhaps by magically transposing "Jesus"' sinless nature on other human beings.
Finally, the morality of Christianity is alien to modern justice systems. No moral rule in the Bible is echoed in the laws of any Western country. Religious believers are aliens to the Western values and the modernity of the countries in which they live.