1 John 4:2

Hereby know ye the Spirit of God
This is a rule by which believers may know whether a man professing to have the Spirit of God, and to be called and sent by him, and whether the, doctrine he preaches, is of him or not:

every spirit that confesseth that Jesus Christ is come in the flesh,
is of God;
or of the Spirit of God; that is, every doctrine which carries this truth in it; or every man that owns, and professes, and publishes this doctrine concerning Christ, is on the side of God and truth; and which contains several articles in it, respecting the person and office of Christ; as that he existed before he came in the flesh, not in the human nature, or as man, or as an angel, but as the Son of God, as a divine person, being truly and properly God; so that this confession takes in his divine sonship, and proper deity, and also his true and real humanity; that the Messiah was incarnate, against the Jews, and was God and man in one person; and that he was really man, and not in appearance only, against the heretics of those times: and it also includes his offices, as that Jesus of Nazareth was the Christ, the Messiah, which the Jews denied, and that he was the anointed prophet, priest, and King; and so is a confession or acknowledgment of all the doctrines of the Gospel, which came by him, as a prophet; and of his satisfaction, sacrifice, and intercession, as a priest; and of all his ordinances and commands as a King; and that he is the only Saviour and Redeemer of men. Now, whoever owns and declares this system of truth, "is of God"; not that everyone that assents unto this, or preaches it, is born of God; a man may believe, and confess all this, as the devils themselves do, and yet be destitute of the grace of God; but the spirit, or doctrine, which contains these things in it, is certainly of God, or comes from him; or whoever brings these truths with him, and preaches them, he is, so far as he does so, on the side of God and truth, and to be regarded.