In his book Forgive, Tim Keller tells the story of a friend of his who was a PhD student at Yale. Keller’s friend once told him that modern people think about slavery and say, “How could people have ever accepted such a monstrosity?” Keller continues:

My friend said, “That’s not the way historians think. They ask: considering the fact it was universally believed by all societies that we had the right to attack an enslaved, weaker people, and since everybody had always done it, the real historical question is, why did it occur to anybody that it was wrong? Whoever first had that idea?”

My friend then answered his own question, pointing out that the first voices in the fourth, 17th, 18th, and 19th centuries who called for the abolition of slavery were all Christians. And the Christian, who called for this justice, believed there was a God of love, who demanded that we love our neighbors—all our neighbors—as ourselves.