12 There remains then the test by which it is to be proved to be the Spirit of God. He has indeed set down a sign, and this, belike, difficult: let us see, however. We are to recur to that charity; it is that which teaches us, because it is the unction. However, what says he here? “Prove the spirits, whether they be from God: because many false prophets have gone out into this world.” Now there are all heretics and all schismatics. How then am I to prove the spirit?
He goes on: “In this is known the Spirit of God.” Wake up the ears of your heart. We were at a loss; we were saying, Who knows? Who discerns? Behold, he is about to tell the sign. “Hereby is known the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is of God: and every spirit that confesses not that Jesus Christ has come in the flesh is not of God: and this is the antichrist, of whom you have heard that he should come; and even now already is he in this world.” Our ears, so to say, are on the alert for discerning of the spirits; and we have been told something, such that thereby we discern not a whit the more.
For what says he? “Every spirit that confesses that Jesus Christ came in the flesh, is of God.” Then is the spirit that is among the heretics, of God, seeing they “confess that Jesus Christ came in the flesh”? Aye, here perchance they lift themselves up against us, and say: You have not the Spirit from God; but we confess “that Jesus Christ came in the flesh:” but the apostle here has said that those have not the Spirit of God, who confess not “that Jesus Christ came in the flesh.”
Ask the Arians: they confess “that Jesus Christ came in the flesh:” ask the Eunomians; they confess “that Jesus Christ came in the flesh:” ask the Macedonians; they confess “that Jesus Christ came in the flesh:” put the question to the Cataphryges; they confess “that Jesus Christ came in the flesh:” put it to the Novatians; they confess “that Jesus Christ came in the flesh.” Then have all these heresies the Spirit of God? Are they then no false prophets? Is there then no deception there, no seduction there? Assuredly they are antichrists; for “they went out from us, but were not of us.”
Source: Homilies on the First Epistle of John (New Advent)