It is hard for us to realize this today, but when Christianity first arose in the world it was not called a religion. It was the non-religion. Imagine the neighbors of early Christians asking them about their faith. “Where’s your temple?” they’d ask. The Christians would reply that they didn’t have a temple. “But how could that be? Where do your priests labor?” The Christians would have replied that they didn’t have priests. “But … but,” the neighbors would have sputtered, “where are the sacrifices made to please your gods?”
The Christians would have responded that they did not make sacrifices anymore. Jesus himself was the temple to end all temples, the priest to end all priests, and the sacrifice to end all sacrifices.
No one had ever heard anything like this. So the Romans called them “atheists,” because what the Christians were saying about spiritual reality was unique and could not be classified with the other religions of the world.
Timothy Keller, The Prodigal God: Recovering the Heart of the Christian Faith, 1st ed. (New York: Dutton, 2008), 13–14.