It’s what happened at the very beginning of the church’s life. The church didn’t simply blaze out into the Greco-Roman world saying “Here’s the truth. You must believe it”. They said, “Look — this is what you say, and that’s very interesting as it echoes with what we say; and, if we talk this through, you might find that what you’re saying has a much fuller expression in what we’re saying.
For contrast, here’s what Libanius tells us about the destructive behaviour of early Christian monks (Oratio 30.8):
…this black-robed tribe…hasten to attack the temples with sticks and stones and bars of iron, and in some cases, disdaining these with hands and feet. Then utter desolation follows, with the stripping of roofs, demolition of walls, the tearing down of statues, and the overthrow of altars, and the priests must either keep quiet or die.
However, in the context of scholarship on early Christianity, Williams’ view is not as unusual as one would think. It is paralleled by the romantic and triumphalist writings of several authors, for example here the papyrologist H. I. Bell in his Forwood Lectures for 1952 (Bell 1953: 105, quoted from Frankfurter 2006: 546):
Later paganism…died with a kind of mellow splendour, like a beautiful sunset, but dying it was. It had been conquered by the truer and finer religion, for which it had itself prepared the way, a religion which at last brought the solution of problems which paganism had posed but to which it had found no answer.
H. I. Bell. 1953. Cults and Creeds in Graeco-Roman Egypt. Liverpool: Liverpool University Press.
D. Frankfurter. 2006. “Traditional Cult”, pp. 543-564, in D.S. Potter (ed.) A Companion to the Roman Empire. Oxford: Blackwell.
The Libanius quote is from the 1969 Loeb edition by A.F. Norman.