The virtue of justice
If I had to choose the one rational virtue that is the most maligned in Christianity, it would definitely have to be justice. This may seem like a counter-intuitive choice, but look at the basic premises of Christianity and how they oppose justice.
Original sin- We are all responsible for the crimes of our first ancestors, crimes that they didn't even know they were committing.
"Jesus"- God incarnates itself as a man and then sacrifices himself in order to somehow mitigate this responsibility.
Heaven and Hell- Eternal reward or punishment for finite crimes. This is usually justified by stating that any sin is a crime against an infinite God, which deserves infinite punishment.
The whole point of being a Christian is to believe in one travesty of justice (sacrificial salvation) in order to escape another (Original Sin). If Christianity is anything at all, it is an all-out attack against justice. If it means anything at all to be a Christian, it is to reject justice. And yet they whine that without God there is no ultimate justice in the universe! As far as Christian projections go, this one takes the cake.
Justice is the commitment to evaluate other people and act accordingly. As a moral agent, I am responsible for making sure that I associate with people who act benevolently (i.e. who consider me as a potential trading partner), and that I do not associate with people who do not. To do so, I must use my rational judgment and treat every individual as a fellow human being, not as a race, religion, social class, status, political group, or as a stepping stone for my own salvation.
What does Christianity preach? "Love your neighbour" and "don't judge lest ye be judged". In short, total non-confront. Why do they teach non-confront? Because their religion is based on submission and faith, and you can't very well be submissive or faithful if you keep evaluating everything for yourself. A belief system which is collectivist must necessarily be anti-justice as well.
As David Kelley says on the subject:
Justice is to society as rationality is to reality in general. It consists in identifying the facts about people and their actions, and evaluating them in terms of their effect on one’s life. This means rating people and their actions based on one’s own hierarchy of values.
Although one should be cautious about pronouncing sweeping moral judgments, one does need to form clear, if somewhat provisional, judgments of the people one deals with on a normal basis. The basis for such judgments should naturally be the facts that are relevant to one’s relationship with the other person. For example, the personal life of one’s banker may not be relevant to how well he handles one’s savings... But it is a vital necessity that one have good reasons to think he is of a rational, dependable, honest character in his financial dealings. Usually, one can reach a reasonable judgment in such cases based on his manner, reputation, track record, ideas and so on.
Logical Structure of Objectivism, p207 and 211
At a basic level, we all do this, perhaps unconsciously- even the most irrational Christian does. Indeed, fundamentalists tend to be very judgmental, although their judgment is not based on facts. But if we start applying this principle consistently, we find that many religious people aren't so great for one's values. What are we to make of people who can do nothing but deceive, brainwash children, and treat others as tools of salvation? This is an issue that all first generation atheists must confront.
By demanding that we non-confront other people's qualities and flaws, the moral system of Christianity ensures that its own peddlers of evil are revered without question. It also ensures that the family structure and the state structures will be revered without question. Non-confront is what keeps society stagnant.
So why is justice a virtue? I think this has been made clear by the discussion so far. Justice is a virtue because there are all kinds of people in a society, and we need to determine who can help us fulfill our values and who cannot, or can even hurt our values. Failing to make such a distinction means that we will be less able to effect our values. This is a pretty easy virtue to recognize.
Well, this is the last of the really anti-Christian virtues that I can see, so this is probably my last entry like this. If anyone has any suggestions, I'd be willing to make more.