In the house of God, in the Church of Christ, the dwellers are of one mind and endure to the end in concord and simplicity. Therefore it was that the Holy Spirit came in the form of a dove, a simple and joyous creature, not bitter with gall, not savage in its bite, not fierce with rending talons. Doves love to find a resting place with men; they cling to association with one house; when they have young they bring them forth together; when they go abroad they keep close in their flight; they spend their lives in mutual and friendly intercourse; they recognize the harmony of peace by the kisses of their beaks; in all things they fulfill the law of unanimity. This is the simplicity that we should know in our Church, this is the charity to which we should attain. ... What part in a Christian breast has the fierceness of wolves, the rage of dogs, the deadly poison of serpents, and the bloody cruelty of wild beasts? We may be thankful when such men as these are separated from the Church, so that they cannot prey upon Christ's doves and sheep, or infect them with the contagion of their cruelty and venom. ... These are they whom the Apostle John smites with a curse, saying, "They went forth from us, but they were not of us. If they had been of us, with us they would have stayed."
Saint Cyprian of Carthage (+258)
On the Unity of the Catholic Church