13 What are we to do then? By what to discern them? Be very attentive; let us go together in heart, and knock. Charity herself keeps watch; for it is none other than she that shall knock, she also that shall open: anon you shall understand in the name of our Lord Jesus Christ. Already you have heard that it was said above, “Whoso denies that Jesus Christ has come in the flesh, the same is an antichrist.” There also we asked, Who denies? Because neither do we deny, nor do those deny.
And we found that some do in their deeds deny; and we brought testimony from the apostle, who says, “For they confess that they know God, but in their deeds deny Him.” Thus then let us now also make the enquiry in the deeds not in the tongue. What is the spirit that is not from God? That “which denies that Jesus Christ has come in the flesh.” And what is the spirit that is from God? That “which confesses that Jesus Christ has come in the flesh.” Who is he that confesses that Jesus Christ has come in the flesh?
Now, brethren, to the mark! let us look to the works, not stop at the noise of the tongue. Let us ask why Christ came in the flesh, so we get at the persons who deny that He has come in the flesh. If you stop at tongues, why, you shall hear many a heresy confessing that Christ has come in the flesh: but the truth convicts those men. Wherefore came Christ in the flesh? Was He not God? Is it not written of Him, “In the beginning was the Word, and the Word was with God, and the Word was God?” Was it not He that did feed angels, is it not He that does feed angels?
Did He not in such sort come hither, that He departed not thence? Did He not in such sort ascend, that He forsook not us? Wherefore then came He in the flesh? Because it behooved us to have the hope of resurrection shown unto us. God He was, and in flesh He came; for God could not die, flesh could die; He came then in the flesh, that He might die for us. But how died He for us? “Greater charity than this has no man, that a man lay down his life for his friends.” Charity therefore brought Him to the flesh.
Whoever therefore has not charity denies that Christ has come in the flesh. Here then do you now question all heretics. Did Christ come in the flesh? “He did come; this I believe, this I confess.” Nay, this you deny. “How do I deny? You hear that I say it!” Nay, I convict you of denying it. You say with the voice, deniest with the heart; sayest in words, deniest in deeds. “How,” do you say, “do I deny in deeds?” Because the end for which Christ came in the flesh, was, that He might die for us.
He died for us, because therein He taught much charity. “Greater charity than this has no man, that a man lay down his life for his friends.” You have not charity, seeing you for your own honor dividest unity. Therefore by this understand ye the spirit that is from God. Give the earthen vessels a tap, put them to the proof, whether haply they be cracked and give a dull sound: see whether they ring full and clear, see whether charity be there. You take yourself away from the unity of the whole earth, you divide the Church by schisms, you rend the Body of Christ.
He came in the flesh, to gather in one, you make an outcry to scatter abroad. This then is the Spirit of God, which says that Jesus has come in the flesh, which says, not in tongue but in deeds, which says, not by making a noise but by loving. And that spirit is not of God, which denies that Jesus Christ has come in the flesh; denies, here also, not in tongue but in life; not in words but in deeds. It is manifest therefore by what we may know the brethren. Many within are in a sort within; but none without except he be indeed without.
Source: Homilies on the First Epistle of John (New Advent)