1 John 4:3
And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
And every spirit that confesseth not,.... The proper deity and sonship of Christ, his true and real humanity, and his Messiahship; or any of his offices, doctrines, and ordinances; or his satisfaction and righteousness; or that peace, pardon, justification, life, and salvation, are by him; all which are meant by what follows,
that Jesus Christ is come in the flesh: this clause is left out in the Ethiopic version, and that without hurting the sense, since it is easily supplied from the preceding verse; and the Alexandrian copy, and the Vulgate Latin version, only read "Jesus": and the latter reads the whole thus, "and every spirit that dissolves Jesus"; that separates the two natures, human and divine, in him, and makes two persons of them; or denies either of them, either that he is truly God, or really man, or denies him to be Jesus, the Saviour; who, as much as in him lies, destroys his person, office, and work, and makes void his obedience, sufferings, and death:
is not of God; neither he nor his doctrine are of God; his doctrine cannot come from God, being contrary to the word of God; and he himself is neither born of God, nor on his side.
and this is that spirit of antichrist: who is against Christ, or opposes himself to him; as he who denies his sonship, his deity, his humanity, his offices, and his grace, manifestly does; every doctrine that is calculated against these truths is the spirit and doctrine of antichrist:
whereof you have heard that it should come, and even now already is it the world; in the false teachers, the forerunners of antichrist;
See Gill on "1 John 2:18".