“The left has no concept of forgiveness of sins,” tweeted conservative Christian Erick Erickson in August during the kerfuffle around the New York Times “1619 Project” on the legacy of American slavery. This is basically Nelson’s argument too. But he pursues it brilliantly at a rather higher level of discourse.

Nelson opens his book by placing Rawls’s recently discovered Princeton University senior thesis, written in 1942, in the long Augustinian tradition of Christianity that denied that sinful humans could save themselves. For Augustine and his followers, Pelagianism—named after a late-antique theologian who was condemned as a heretic by the Catholic Church—overstated the extent to which human beings can earn their salvation. Such a belief verged on an ideology of self-redemption of individual sinners or of humanity itself that (as Rawls put it at age twenty) “rendered the Cross of Christ to no effect.” For Rawls, at the time a committed Christian who planned a career in the Episcopal priesthood before World War II service in the Pacific caused him to lose his faith, it followed that “no man can claim good deeds as his own.” To contend otherwise inflated human capacity and courted sacrilegious idolatry of humanity itself.

Nelson contends that this Augustinian response to Pelagianism lurked in Rawls’s defense of fair distributional justice long after he had moved on to secular philosophy. In A Theory of Justice, Rawls remarked that “no one deserves” their social ascendancy and the natural gifts—intelligence or industriousness—with which they achieved it. The fact that one person was endowed with them and another not was “morally arbitrary.” A theory of justice aiming at fairness rather than fortune would reject any sense that people deserved their class position. Some redistribution from the rich to the rest was therefore just.

What Nelson does with this parallel between Rawls’s Christian senior thesis and his mature theory of redistribution is more contentious. Demonstrating that most founders of the liberal tradition were Pelagians, he insists that it is difficult to reconcile Rawls’s rejection of moral arbitrariness with the politics he hoped to advance.

“Liberalism,” writes Nelson, “began as a theodicy.” By this he means that for the major liberal thinkers in the early-modern period, the attempt to justify the ways of God to men almost always included the belief that God is unfailingly good. It is their own autonomy that leads humans, if they choose not to conform to God’s plan, to introduce evil into the world on their own. What made for the correlation of Pelagianism with liberalism is that the theological defense of human freedom—including freedom to err—implied that individuals should be allowed politically to seek perfection on their own, without the interference of states or sects. Liberalism was born out of the insistence that, since agents were free enough to save themselves, they had to be left alone enough to have a chance to do it.

Observing that early liberals embraced the very theology that Rawls rejected, Nelson thinks Rawls’s followers are left with a big problem. Liberalism originated in the Pelagian heresy that refuses to saddle human beings with original sin, or to make them utterly dependent on the divine, but instead grants them autonomy, dignity, and (at least potential) self-made perfection. How, then, can Rawls and his followers reject Pelagianism without also rejecting liberalism?

Nelson’s answer: they can’t. Either you adopt the Augustinian line that, while no one earns their gifts and talents, any seemingly unfair distribution is part of God’s mysterious design, whose meaning is to be revealed only at the end of time; or you adopt the Pelagian view that you do earn them—that greater wealth really might reflect greater merit. You can’t have it both ways, as Rawls and his followers want.

After suggesting that Rawls’s Augustinian case for redistribution is incoherent, Nelson spends the rest of his book arguing that Pelagians, who believe in the autonomy of human beings, will also find it difficult to make a case for more egalitarian distribution—at least as a matter of obligatory justice, as opposed to optional public policy (a possibility Nelson graciously acknowledges).

The gist of his argument is that nobody knows for sure that the distribution of gifts and talents is unjust. Denying that alternative worlds were better, he observes, is what early-modern Pelagians spent their time doing, perhaps most famously when arch-Pelagian G. W. Leibniz coined the term “theodicy.” It is illicit, Leibniz claimed, to infer from the apparent unfairness of the created world that God didn’t do the best he could with the imperfect human materials he was working with. If you think our reality is not already optimal, you haven’t considered what God had to deal with in making it. What if doing better on one front might have worsened the world in other ways?

What the Lisbon earthquake was to Leibnizians—a scandal for the simpleminded that did not necessarily disprove God’s goodness and justice—neoliberalism is to Nelson. How, he wonders, do progressive advocates of redistribution know we are not already in the fairest of all possible worlds? The mere fact of unequal distribution—whether of wealth itself or of the talent it rewards—hardly proves that a better state of affairs is possible, or that the pursuit of a more equitable distribution would not lead to a worse outcome.

But Nelson’s argument could be turned against him: if no one can know for sure that we are not already in the fairest of all possible worlds, neither can anyone know that we are. Leibniz himself conceded that no conclusive proof is available that the way the world has worked out is best. “Theodicy,” Nelson says when resting his own case, “could be neither demonstrated nor refuted.” If that is so, however, then it is really a matter of determining whether to force on egalitarians the obligation to prove conclusively that—in the old slogan of the alter-globalization movement—another world is possible. Why not instead place the burden on those who conclude that existing inequality is the best available scenario?

In his responses to Leibniz, in any case, Voltaire never took it upon himself to prove that his foe had rationalized horror, the better to preserve the belief in God’s goodness in a flawed world. Rather, Voltaire simply ridiculed him. However difficult if might be to show that it’s false, the notion that our history of crimes and misfortunes has led to the best imaginable society is simply too incredible for us to allow it to get in the way of a zeal for just reform. Nelson surely wouldn’t have required of abolitionists that they prove to dominant skeptics that a better world without chattel slavery was possible before they resolved to achieve it. Why is the case of fair distribution any different in the alarmingly unequal situation of the present?

But there is also a deeper quandary about the way that Nelson brings old theology to bear on contemporary philosophy. He insists that a lot follows from restoring their lost unity, as he does so intrepidly in his book.