2 John 1:9
Whosoever transgresseth, and abideth not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son. Whosoever transgresseth, and abideth not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son.
The verse draws a sharp dividing line between two classes: the one who transgresses and does not remain in the doctrine of Christ does not have God, while the one who remains in that doctrine has both the Father and the Son. Remaining is decisive, for to depart from Christ's teaching is to lose God, and to abide in it is to possess fellowship with Father and Son together. That doctrine centers on the person and work of Christ himself, defining who truly belongs to God.
It includes the truth that Jesus Christ came in the flesh. It also includes his divinity. These two truths stand together as the substance of abiding: to confess Christ truly incarnate and truly divine is to remain in his doctrine, and to deny either is to transgress and go beyond it.
Abiding in that doctrine also governs the relation of love, law, and faith. Christ's love does not release believers from obedience to God's law. Obedience, on the other hand, cannot take the place of faith: keeping God's law and Jewish customs cannot save without faith in Christ and his blood. Both errors — making love an excuse for disobedience, and making law-keeping a substitute for trust in Christ's atoning blood — are departures from the doctrine of Christ.
To reject the truth that Christ came in the flesh is to place oneself among the deceivers and antichrists. Such denial is not a minor difference but marks the transgression described in the verse, separating the denier from God.
The verse also gives a rule for church fellowship. Those who pursue a course of transgression must be visited and labored with for repentance. If they refuse to repent, their names must not be retained on the church books but they must be separated from fellowship, lest the saints be held accountable for their evil deeds.