Hereby, or by this, know ye He lays down a special mark by which they might more easily distinguish between true and false prophets. Yet he only repeats here what we have met with before, that as Christ is the object at which faith aims, so he is the stone at which all heretics stumble. As long then as we abide in Christ, there is safety; but when we depart from him, faith is lost, and all truth is rendered void. [1] But let us consider what this confession includes; for when the Apostle says that Christ came, we hence conclude that he was before with the Father; by which his eternal divinity is proved. By saying that he came in the flesh, he means that by putting on flesh, he became a real man, of the same nature with us, that he might become our brother, except that he was free from every sin and corruption. And lastly, by saying that he came, the cause of his coming must be noticed, for he was not sent by the Father for nothing. Hence on this depend the office and merits of Christ. As, then, the ancient heretics departed from the faith, in one instance, by denying the divine, and in another by denying the human nature of Christ; so do the Papists at this day: though they confess Christ to be God and man, yet they by no means retain the confession which the Apostle requires, because they rob Christ of his own merit; for where freewill, merits of works, fictitious modes of worship, satisfactions, the advocacy of saints, are set up, how very little remains for Christ! The Apostle then meant this, that since the knowledge of Christ includes the sum and substance of the doctrine respecting true religion, our eyes ought to be directed to and fixed on that, so that we may not be deceived. And doubtless Christ is the end of the law and the prophets; nor do we learn anything else from the gospel but his power and grace.

Footnotes

1 - It appears that by "spirit" throughout this passage, we are to understand a teacher claiming, rightly or falsely, to be influenced by God's Spirit. Nor would it be improper, but suitable to the context, to consider the spirit of God in this verse as meaning a teacher guided by God. The meaning of the passage might be thus expressed, -- 2. "By this know ye the teacher of God; every teacher who confesses Jesus Christ as having come in the flesh, is from God; and 3. every teacher who does not confess Jesus Christ as having come in the flesh, is not from God; and this is the teacher of Antichrist, (or, the Anti Christian teacher,) of whom ye have heard that he is coming, and he is now already in the world." -- Ed

Hereby - Greek, "By this;" that is, by the test which is immediately specified.
Know ye the Spirit of God - You may discern who are actuated by the Spirit of God.
Every spirit - Everyone professing to be under the influence of the Spirit of God. The apostle uses the word "spirit" here with reference to the person who made the claim, on the supposition that everyone professing to be a religious teacher was animated by some spirit or foreign influence, good or bad. If the Spirit of God influenced them, they would confess that Jesus Christ had come in the flesh; if some other spirit, the spirit of error and deceit, they would deny this.
That confesseth - That is, that makes a proper acknowledgment of this; that inculcates this doctrine, and that gives it a due place and prominence in his instructions. It cannot be supposed that a mere statement of this in words would show that they were of God in the sense that they were true Christians; but the sense is, that if this constituted one of the doctrines which they held and taught, it would show that they were advocates of truth, and not apostles of error. If they did not do this, 1-John 4:3, it would be decisive in regard to their character and claims.
That Jesus Christ is come in the flesh - Benson and some others propose to render this, "That Jesus, who came in the flesh, is the Christ." But this is liable to serious objections.
(1) it is not the obvious interpretation.
(2) it is unusual to say that Jesus "had come in the flesh," though the expression "the Son of God has come in the flesh," or "God was manifested in the flesh," would be in accordance with the usage of the New Testament.
(3) this would not, probably, meet the real point in the case. The thing denied does not appear to have been that Jesus was the Messiah, for their pretending to be Christian teachers at all implied that they admitted this; but that the Son of God was "really a man," or that he actually assumed human nature in permanent union with the divine. The point of the remark made by the apostle is, that the acknowledgment was to be that Christ assumed human nature; that he was really a man as he appeared to be: or that there was a real incarnation, in opposition to the opinion that he came in appearance only, or that he merely seemed to be a man, and to suffer and die. That this opinion was held by many, see the Introduction, Section III. 2. It is quite probable that the apostle here refers to such sentiments as those which were held by the "Docetae;" and that he meant to teach that it was indispensable to proper evidence that anyone came from God, that he should maintain that Jesus was truly a man, or that there was a real incarnation of the Son of God. John always regarded this as a very important point, and often refers to it, John 19:34-35; John 20:25-27; 1-John 5:6. It is as important to be held now as it was then, for the fact that there was a real incarnation is essential to all just views of the atonement. If he was not truly a man, if he did not literally shed his blood on the cross, of course all that was done was in appearance only, and the whole system of redemption as revealed was merely a splendid illusion. There is little danger that this opinion will be held now, for those who depart from the doctrine laid down in the New Testament in regard to the person and work of Christ, are more disposed to embrace the opinion that he was a mere man; but still it is important that the truth that he was truly incarnate should be held up constantly before the mind, for in no other way can we obtain just views of the atonement.
Is of God - This does not necessarily mean that everyone who confessed this was personally a true Christian, for it is clear that a doctrine might be acknowledged to be true, and yet that the heart might not be changed; nor does it mean that the acknowledgment of this truth was all which it was essential to be believed in order that one might be recognised as a Christian; but it means that it was essential that this truth should be admitted by everyone who truly came from God. They who taught this held a truth which he had revealed, and which was essential to be held; and they thus showed that they did not belong to those to whom the name "antichrist" could be properly given. Still, whether they held this doctrine in such a sense, and in such connection with other doctrines, as to show that they were sincere Christians, was quite another question, for it is plain that a man may hold and teach the true doctrines of religion, and yet have no evidence that he is a child of God.

Hereby know ye the Spirit of God - We know that the man who teaches that Jesus Christ is the promised Messiah, and that he is come in the flesh, is of God - is inspired by the Divine Spirit; for no man can call Jesus Lord but by the Holy Ghost.

(2) Hereby know ye the Spirit of God: (b) Every spirit that confesseth that (c) Jesus Christ is come in the (d) flesh is of God:
(2) He gives a certain and perpetual rule to know the doctrine of antichrist, that is, if either the divine or human nature of Christ, or the true uniting of them together is denied: or if the least jot that may be, be detracted from his office who is our only king, prophet and everlasting high priest.
(b) He speaks simply of the doctrine, and not of the person.
(c) The true Messiah.
(d) Is true man.

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.

Hereby--"Herein."
know . . . the Spirit of God--whether he be, or not, in those teachers professing to be moved by Him.
Every spirit--that is, Every teacher claiming inspiration by the HOLY SPIRIT.
confesseth--The truth is taken for granted as established. Man is required to confess it, that is, in his teaching to profess it openly.
Jesus Christ is come in the flesh--a twofold truth confessed, that Jesus is the Christ, and that He is come (the Greek perfect tense implies not a mere past historical fact, as the aorist would, but also the present continuance of the fact and its blessed effects) in the flesh ("clothed with flesh": not with a mere seeming humanity, as the DocetÃ&brvbr; afterwards taught: He therefore was, previously, something far above flesh). His flesh implies His death for us, for only by assuming flesh could He die (for as God He could not), Hebrews 2:9-10, Hebrews 2:14, Hebrews 2:16; and His death implies His LOVE for us (John 15:13). To deny the reality of His flesh is to deny His love, and so cast away the root which produces all true love on the believer's part (1-John 4:9-11, 1-John 4:19). Rome, by the doctrine of the immaculate conception of the Virgin Mary, denies Christ's proper humanity.

Every spirit - Or teacher. Which confesseth - Both with heart and voice. Jesus Christ, who is come in the flesh, is of God - This his coming presupposes, contains, and draws after it, the whole doctrine of Christ.