1 John 4:3

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).