The Context of 2 John 9
There are those who maintain that the expression "doctrine of Christ" in 2 John 9 does not mean Christ's doctrine. They affirm that it means doctrine about Christ. They say that from the standpoint of grammar, the expression could mean either Christ's doctrine or doctrine about Christ, but contend that the context proves that when John used the expression he meant doctrine about Christ.
It is my conviction, however, that the context does not support their conclusion., I believe that the context indicates that when John used the expression "doctrine of Christ," he meant Christ's teaching. It is Christ's teaching that John stressed from the beginning of the letter.
Note the emphasis placed on truth (Christ's teaching) in verses 1-2. Then in verse 4 John expressed his joy that the elect lady's children were "walking in truth" (abiding in Christ's teaching). Again in verse 6 John stressed the importance of walking after His commandments. Thus, the point that was uppermost in John's mind when he penned this letter was walking in truth (abiding in Christ's teaching).
Those who maintain that the expression "doctrine of Christ" in verse 9 means doctrine about Christ point to verse 7. They observe that according to verse 7 the particular error John had in mind was that of the deceivers who denied that Christ had come in the flesh. This error John mentioned in verse 7 was erroneous teaching about Christ; thus, it is concluded that John must have meant doctrine about Christ in verse 9, referring only to the one specific error named in verse 7.
It is true that the error that was a particular threat when John wrote was the error mentioned in verse 7. However, the teaching of this particular error was but one specific instance of not abiding in truth. The general principle of abiding in Christ's teaching is that which John stressed throughout the letter, and verse 7 simply mentions one particular violation of that principle.
Truth Magazine, XVIII:33, p. 13