2 John 1:7-9

⁷ For many deceivers have gone out into the world who do not confess Jesus Christ as coming in the flesh. This is a deceiver and an antichrist.

⁸ Look to yourselves, that we do not lose those things we worked for, but that we may receive a full reward.

⁹ Whoever transgresses and does not abide in the doctrine of Christ does not have God. He who abides in the doctrine of Christ has both the Father and the Son.

For (v.7) — This gives one great reason … for the preceding instruction as to true Christian love, walking “in truth” and after the Lord’s commandments. All this provides a safeguard against error, and its deceptive propagators. The word rendered “deceivers” signifies imposters of the vagabond type, and hence any kind of corrupter. … It seems likely that these seducers had been assembling with the churches and had gone forth from them spreading their evil doctrines in the world. … Compare what the Apostle says about the antichrists in the 1 John 2:18-19; “they went out from us, but they were not of us”; and now he speaks similarly of one who teaches the special error mentioned. — Vine, page 116.

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do not confess (v.7) — Not to confess is the same thing as denying. … The denial was a denial both of the fact of the Incarnation and of its possibility at any time, including the future Coming when we shall see Him “even as He is” (1 John 3:2). The rejection of the possibility that God could become manifest in the flesh shows traces of Gnosticism. … Hence arose these deceivers who taught that matter is essentially evil, and therefore God could not associate Himself with evil flesh by incarnation, and that Christ was indeed Divine, but His body was a phantom. — Vine, pages 116-117.

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“This is” (v.7) sets a definite stamp upon the errorist. Whatever additional characteristics there are of teachers of this false doctrine, this is decisive. In the capacity of “the deceiver” the evil teacher acts against man; in his capacity as “the antichrist” he acts against God. An antichrist as a precursor of the coming Antichrist, is not only opposed to Christ, he aims at usurping His authority. — Vine, page 117.

Look to yourselves (v.8) — self examination

transgresses (v.9) — The Greek means “goes beyond” the teaching respecting Christ. … All who reject a Divine and atoning Savior reject God. Only they who accept the teaching of the Holy Spirit respecting the Person and Work of the Lord Jesus Christ know God and possess Him. all who reject His Godhead and atonement have a god of their own imagination; and that god is an idol. — Williams, page 1018.

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transgresses (v.9) — “Goes Onward” was a sarcastic reference to the professed higher knowledge of the false teachers. … The significance of the word, rightly rendered “goes onward,” is not that of progress itself, but an advance in teaching that goes beyond the teaching of Christ; that is to say, not only the teaching concerning the Person of Christ, but that which He gave and commanded his followers to teach. Anyone who goes beyond this, the Apostle declares, “does not have God” (i.e., as his God), however much he may claim to know Him. … There may be great professions of religion and religious zeal, but to add to, or depart from, the teaching of  Christ (as Jude calls it, “the faith once for all delivered to the saints”) must eventually involve that condemnation which He foretold when He said, “Then will I profess unto them, I never knew you; depart from Me, ye that work iniquity” (or rather, “lawlessness”; that is whatever is contrary to the will of God. — Vine, page 118.