It is obvious from this passage that "the doctrine of Christ" is vitally important, for a person who does not "abide" (that is "continue") in this doctrine does not know God at all. In fact, the next verse forbids fellow Christians even to welcome such a person into one's home or into the fellowship of his church. That person certainly must not be permitted to teach his false doctrine there. "If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed" (2 John 10).
But exactly what is "the doctrine of Christ"? In context, it must certainly include the incarnation of God in Christ in human flesh (2 John 7). In addition, however, the Greek word actually means "the teachings," and probably refers to everything the Scriptures teach concerning Him, as well as all that He Himself taught. This breadth of meaning in the Word is illustrated by the Lord's warning against "the doctrine of Balaam" (Revelation 2:14), which obviously refers to Balaam's teachings. The "doctrine of Christ" thus embraces everything God's Word teaches about Christ's person and work, as well as what He Himself taught about everything else.
There may be questions that can arise about interpretation or proper translation of some portion of the doctrine of Christ, but not about its validity or authority. Once the teaching is clearly understood, it must not be questioned or diluted or explained away, but simply believed, practiced, and proclaimed. For any teacher that "abideth not in the doctrine of Christ, hath not God" and we must not have Christian fellowship with him or encourage his erroneous teaching in any way. HMM