It is difficult to date when western culture stopped being intellectually Christian. The most probable date is the Enlightenment, though not because of any of the prominent atheists of the day such as Voltaire or Hume. Atheism has never been a threat to Christianity; it is too esoteric for people who are not interested in their religion, and too inhuman for people who are. The enlightenment was, rather, the first major intellectual wave following the earthquake of the reformation.
Like an earthquake the reformation was a great shattering, and for the first time several religions took hold within christendom. Even if all of the protestant faiths had accepted reason, rather than making war with it as most of them did, the strain of so many religions vying for the minds of europeans (and later Americans) inevitably tired them of thinking about religion. The Enlightenment was the first symptom of this great exhaustion. When people lack the energy for religion, religion leaves the public sphere and becomes private. When religion becomes private it becomes varied. There's nothing new in this; the world has had many cosmopolitan cultures before Christianity where private religions grew like wildflowers. What's strange about the modern flowering of private religions is that none of them shake Christianity, however little they believe in it.
C.S. Lewis once said that Jesus was rather unremarkable as a moralist, and so far as he meant it, this is true. Jesus espoused a moral theory that you might find almost anywhere. But morality is principles applied to circumstances. Christians do not differ much in their theory of justice. Christians do differ greatly in what they believe the circumstances are. But to make this clear, I first have to explain what justice and mercy are, and what christianity is all about, since ideas of these things are very muddled in modern discussions.
Justice consists of giving to a man what he is owed, and him giving what he owes. A man who's had his ox stolen is owed his ox by the theif, and a man who kills his brother owes his life to his brother. Justice is balance. "An eye for an eye, a tooth for a tooth" is the very essence of justice.
Mercy, by contrast, is a man forgiving debts that are owed to him. Mercy is imbalance.
Nearly everyone everywhere has always approved of justice. Few people have thought meanly of mercy, but the problem with mercy is that there's rarely a practical case to be made for it. Mercy contains within itself no reward. Indeed, mercy often seems like rewarding evil. If a man rapes a woman and she forgives him rather than prosecutes him as is her right, it is not obvious how this will discourage him from raping her again.
Before moving on, I should probably qualify how this is different from the forgiveableness of crimes. It is obvious that some crimes are so small that the cost of punishing the malefactor is greater than the crime; some crimes are so small that, punishment being the blunt tool that it is, any punishment would be excessive. When a child steals a piece of candy or some other very small thing, this is forgiveable because the hurt is so small. This isn't a matter of mercy but of practicality. Some sins are so venial that they're not worth much human time or concern. That's a different thing entirely than forgiving an offense for which one has the time and energy to prosecute
Christianity is of course too rich and complex to be described simply, but for all that the point of Christianity is Jesus, the annointed one of God. The Son of God became a man, lived among us, was tortured to death unjustly, and rose from the dead like he will raise us after death as well.
There are more implications to Jesus death than a lifetime of study can comprehend, but relevant to the moment is that Jesus, by suffering terribly, paid a debt that he didn't owe. Much is made of how all sin drives a wedge between us and God, and how Jesus paid the price to reconcile us with God. But he also paid the price to reconcile us with each other. Everyone who kills a person, Jesus offers his death as the price for. In a way Jesus death is God's apology to us for the injustices we suffer in his creation. Equally, Jesus death is our own apology to our fellow creatures for the injustices we inflict on them, because Jesus has offered to pay what we owe on our behalf.
This offer, freely given by God, is given on God's terms. When Jesus says, "judge not, lest you be judged also," he explains the deal. God will show us the mercy we show to others. This makes mercy not just Godly, but highly practical. Forgiving a man for stealing my wallet buys my forgiveness for when I was mean to my neighbor who wanted a ride to the supermarket. I might forgive a theif for better reasons, but if I forgive him because God paid the price for him, we will still be reconciled. And the central theme of Christianity is that all is well that ends well.
(I should note in passing that forgiving a thief for stealing is not the same thing as keeping one's door unlocked at night.)
The result is that within a christian framework, it makes sense to forgive everyone for everything, and to withhold judging anyone. We withold judgment in an exchange — we barter for our own forgiveness. This may perhaps sound a bit crass. We are taught to be good for goodness' sake. Yet this isn't a question of goodness. God is offering us a deal. Whether we accept it is not a moral question but a practical question. We would be within our rights to reject God's offer, just as we'd be within our rights to reject an offer to purchase a posession of ours for a thousand times what we paid for it. It would be very foolish, but it would not be wrong.
The trade of infinite forgiveness and withholding judgment don't make sense outside of a Christian framework because there's nobody to make the trade with. If you are an agnostic and I've killed your brother, you don't believe that anyone has offered to pay for my crime in my place. As a murderer, I'd obviously be an evil man, and no one has offered to keep from calling you bad for your misdeeds if you keep from calling me bad for mine. (In case there's any chance of confusion, I don't mean that God is concerned with name-calling, but with categorization; if you judge me that I'm evil, you won't admit me into your society, nor help me when I need help.) The agnostic and the christian both hold the same theory of justice. They different only about what's already been paid.
The strange thing is that the modern agnostic will still likely believe in forgiveness and withholding judgement, because he grew up in the Christian tradition and was raised to believe in them. He cannot account for them — he could not talk another agnostic who doesn't believe in these things into believing in them. But it is human nature to try to make sense of what we believe, and modern agnostics' attempts to make their beliefs understandable produce a very great confusion of thought.
Attempting to make sense of mercy, modern agnostics tend to either minimize crimes, or ignore punishments. The simplest expedient to simulating mercy is to expand as far as possible the list of crimes which are forgiven because they are small. The anonymity created by the modern population sizes, wealth, large corporations, and beautocratic systems allows a great many crimes to have no obvious victim, and thus to be excusable in an imitation of mercy. For those crimes which are not forgiveable, your typical agnostic simply doesn't think about punishments. They talk about what should be illegal without any reference to what actually happens to people who are convicted of crimes; making something a crime with a large punishment is only a way of expressing great dissaproval of the crime. The criminal is altogether forgotten. Whether this is better for the criminal, it at least keeps the person outlawing his crime from passing any judgement on him as a person.
Another example of this muddled mis-application is the idea of tolerance. Jesus ate with tax collectors and with sinners because his love overflowed so fully that he loved all people despite their sins. Tax collectors collaborated with the roman government to, in essence, steal a lot of money from poor people. They were despised with good reason, because they were powerful thieves who preyed on the weak. Yet Jesus at and drank with them, knowing full well of their wicked ways. Jesus could forgive them, because he was going to made amends for all of their wickedness. Their sins were not forgiveable, but they were, because their sins were to be paid for, and Jesus loved them enough to pay for their sins, and call them to reconciliation with their fellow creatures. If tolerance was ever a virtue, this was the virtue of tolerance.
Modern agnostics still believe that tolerance is a virtue, but since they don't believe that wickedness has been paid for, they can't actually tolerate anything that they disapprove of. Their method, instead, is to pretend that everyone is good. They tolerate people who are different by the simple expedient of disbelieving in differences. If you point out that some cultures don't believe in helping strangers, they'll call you intolerant, because by their definition you are. You're trying to claim something which must necessarily lead them to think ill of the people in those cultures, and thinking ill of people is bad.
The same is true of a great many of the christian virtues as they're practiced in western culture today. Not believing in the same set of circumstances as christians, they can't apply basic moral principles to come to the same conclusions. Yet the feel both the principles and the conclusions so strongly that they are forced to invent a set of circumstances which allow them to come to this conclusion. They're upside down, and so to keep their heard in the right place, they have to twist their head into great contortions.
And this is perhaps to their credit. After all, only a few saints were also philosophers. But the main problem with this setup is that it's unstable. If one's mercy depends on pretending that most crimes are forgiveable and tolerance depends on pretending that all people are the same, these things will not long outlast contact with the real world.
Even that may be for the best, eventually. It's easier to convert pagans to Christianity than it is to convert muddled pseudo-christians who can't understand Jesus' sacrifice because they won't understand anyone's sin. It was Chesterton ,I think, who said that most vices are just virtues practiced monomanaically. That is, vices are just virtues practiced without understanding. That at least characterises the vices of our own age: it's the age of misunderstood virtues.