Augustine of Hippo, on 1John 4:3

“And every spirit that dissolveth Jesus, is not of God: and this is Antichrist, of whom you have heard that he cometh, and he is now already in the world.”

“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 denieth that Jesus Christ is come in the flesh." And what is the spirit that is from God? That "which confesseth that Jesus Christ is come in the flesh." Who is he that confesseth that Jesus Christ is 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 is come in the flesh. If thou stop at tongues, why, thou shalt hear many a heresy confessing that Christ is come in the flesh: but the truth convicteth 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 doth 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 hath 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 is come in the flesh. Here then do thou now question all heretics. Did Christ come in the flesh? "He did come; this I believe, this I confess." Nay, this thou deniest. "How do I deny? Thou hearest that I say it!" Nay, I convict thee of denying it. Thou sayest with the voice, deniest with the heart; sayest in words, deniest in deeds. "How," sayest thou, "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 hath no man, that a man lay down his life for his friends." Thou hast not charity, seeing thou for thine 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. Thou takest thyself away from the unity of the whole earth, thou dividest the Church by schisms, thou rendest the Body of Christ. He came in the flesh, to gather in one, thou makest an outcry to scatter abroad. This then is the Spirit of God, which saith that Jesus is come in the flesh, which saith, not in tongue but in deeds, which saith, not by making a noise but by loving. And that spirit is not of God, which denies that Jesus Christ is 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.”

Imported from an open dataset — not yet checked against the printed edition.