"Social justice" is a misnomer, for what is "just" about giving the poor more than they have earned? What we really mean is "social mercy": in this world, all men are not created equal, and they are certainly not given equal means-- rather, they are made more equal by the application of mercy. "Social justice" would mean letting nature run its course; letting humanity evolve by the survival of the fittest, giving people exactly what they deserve: nothing. But we don't do that: we heal the sick, build homes for the poor, and help those who can't help themselves, because we believe all human lives have equal value. But how do we justify our longing for social mercy when we do not trust in the God who first showed mercy to us? For if we believe in an impotent or nonexistent god, we are forced by the rules of logic to conclude that our convictions for uplifting the downtrodden are merely vestiges of our meandering, directionless evolutionary past, and thus the value we give life is nothing but an illusion or a by-product: a tumor on humanity's side. But we all live as though dignity is real; indeed it is not possible to deny its reality without lying to one's self. So from there we are only a modus ponens away from concluding one of two things: that either our belief in dignity, and thus our quest for equality, is wholly invalid; such a quest only holds back the evolution of our species, and the only correct option is to let the cold hand of "social justice" run its course -- leaving the poor with nothing, and the despots with whatever they can envelop -- or, that a merciful God exists, and that He mandates our quest for equality by creating us all in His image; and that He gives us greater dignity through the valuation of our souls than we are capable of arbitrarily giving each other.
Or, as C.S. Lewis put it, "My argument against God was that the universe seemed so cruel and unjust. But how had I got this idea of just and unjust?"