A painting by Leonardo da Vinci, circa early 1500s, has just been sold for $450m. That makes it the most expensive painting ever. It is called Salvator Mundi, and depicts Christ, hand raised as if about to give a priestly blessing. He is the saviour of the world, says Leonardo. It was probably only a few years after Leonardo finished this great work that, on the other side of the Alps, a German monk would come up with an idea about this act of salvation that would totally transform the intellectual landscape of Europe.

Generally, the beginning of the Reformation is regarded as being marked by Martin Luther’s posting on the door of All Saints’ Church, Wittenberg, a long list of demands to the Roman church on the subject of its financial corruption. But Luther’s key intellectual breakthrough had occurred several years before. Living as a monk, Luther wrestled with the thought that, despite his rigorous standard of living, nothing he was capable of as a human being would ever be good enough for God. And that, if God was entirely just – that is, if God judged us according to our merits – then all of us are in deep trouble. Hamlet makes the same point in a conversation with Polonius: if you treat everyone as they deserve, “who shall ’scape whipping?”

Later Luther admitted that during this dark period he actually hated God, and hated the idea of a just God especially. His breakthrough, the so-called “Tower experience”, was through the extraordinary idea that God does not treat us fairly at all, but bestows his grace upon human beings gratuitously, far above and beyond what we deserve. In other words, human life before God is not all about us stacking up moral merit points, thus to guarantee our passage to heaven. Protestants disparage this as “works righteousness” and see it as a foolish and impossible task. The only way out of the trap of the human condition is to admit our moral incapacity and call on God for help. There is no way to bully or lobby the divine into doing this. Salvation is top down. And we are as dependent upon Salvator Mundi as we are dependent upon the rain.

Salvation is top down. We are as dependent upon Salvator Mundi as we are dependent upon the rain. Photograph: ART Collection

The upshot of this shift in thinking is that Protestant Christianity is not really all that interested in morality. You can’t get yourself saved by being good. It’s not that morality is a bad thing per se – but that it does not reach into the darker parts of human nature. For what if, deep down, we are all a bit more like the Harvey Weinsteins of this world than we care to admit?

Analysis of the phrases we type into the Google search engine reveals that humanity is much more drawn to pornography and violence than we acknowledge. Morality is an important public-order issue. But there is a vast dark world below the surface of our outward behaviour. As the Christian socialist RH Tawney memorably put it: “The goodness we have reached is a house built on piles driven into black slime.”

One of the thing atheists sometimes complain about is that Christians equate being good with religious faith. Or that Christians think of themselves as somehow morally superior. Thus when a Christian behaves badly, others cry: “Call yourself a Christian?” But this is entirely wrongheaded. If there is a criticism of Christianity, it must surely come from entirely the other direction, that by emphasising our intrinsic darkness, Christianity can be shockingly indifferent to bad behaviour.

Take the ghastly Roy Moore, the former Alabama judge who put up the Ten Commandments in his courthouse and who is now running for election to the US Senate despite widespread allegations of sexual assault on women and girls. Yes, he is a still a Christian. He may, of course, also be a complete and utter shit. But the two things are far from incompatible. Indeed, they are probably intimately connected. For the Christianity of Salvator Mundi is designed specifically for all us shits. And the good news is that it doesn’t cost $450m to find it. You only have to ask. And it only ever comes for free.