7 For hear and see. Certainly all who go out from the Church, and are cut off from the unity of the Church, are antichrists; let no man doubt it: for the apostle himself has marked them, “They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us.” Therefore, whoso continue not with us, but go out from us, it is manifest that they are antichrists. And how are they proved to be antichrists? By lying. “And who is a liar, but he that denies that Jesus is the Christ?” Let us ask the heretics: where do you find a heretic that denies that Jesus is the Christ?
See now, my beloved, a great mystery. Mark what the Lord God may have inspired us withal, and what I would fain work into your minds. Behold, they went out from us, and turned Donatists: we ask them whether Jesus be the Christ; they instantly confess that Jesus is the Christ. If then that person is an antichrist, who denies that Jesus is the Christ, neither can they call us antichrists, nor we them; therefore, neither they went out from us, nor we from them. If then we have not gone out one from another, we are in unity: if we be in unity, what means it that there are two altars in this city?
What, that there are divided houses, divided marriages? That there is a common bed, and a divided Christ? He admonishes us, he would have us confess what is the truth:— either they went out from us, or we from them. But let it not be imagined that we have gone out from them. For we have the testament of the Lord's inheritance, we recite it, and there we find, “I will give You the nations for Your inheritance, and for Your possessions the ends of the earth.” We hold fast Christ's inheritance; they hold it not, for they do not communicate with the whole earth, do not communicate with the universal body redeemed by the blood of the Lord.
We have the Lord Himself rising from the dead, who presented Himself to be felt by the hands of the doubting disciples: and while they yet doubted, He said to them, “It behooved Christ to suffer, and to rise from the dead the third day: and that repentance and remission of sins should be preached in His name” — Where? Which way? To what persons?— “through all nations, beginning at Jerusalem.” Our minds are set at rest concerning the unity of the inheritance! Whoso does not communicate with this inheritance, is gone out.
Source: Homilies on the First Epistle of John (New Advent)