Cyprian of Carthage, Three Books of Testimonies Against the Jews III.88: That believers ought to be simple, with prudence

In the Gospel according to Matthew: “Be ye prudent as serpents, and simple as doves.” And again: “Ye are the salt of the earth. But if the salt have lost his savour, in what shall it be salted? It is good for nothing, but to be cast out abroad, and to be trodden under foot of men.”