John noted his joy in that he found his readers walking in the truth (v. 4). This joy comes from the fact that walking in the truth, or abiding in the “doctrine of Christ,” puts us in fellowship with God, and Jesus (v. 9). On the other hand, he rejoices in what they are not – transgressors of the “doctrine of Christ.” Fellowship with God is severed when one fails to abide in His teaching.

Also, implicitly included within the formula stated before concerning fellowship with God, is our behavior toward other men. Fellowship is not to be extended from one who has fellowship with God to one who does not. To do so would be to contradict the logic concerning fellowship with God, for whoever extends fellowship to one who is not abiding in the “doctrine of Christ” is by extension participating in their evil deeds (v. 10). If God is light, and in Him is no darkness at all (cf. 1 John 1:5-7), we cannot be in fellowship with Him when we choose to venture into the “power of darkness” to associate with those contrary to Him (cf. Colossians 1:13). To be in darkness is to not be in light, and to be in light is to not be in darkness.

After finishing his thoughts concerning walking in truth, the deceivers, and the boundaries of fellowship, John concluded his epistle by an expression of hope to speak further about these things “face to face” (v. 12). This he wished to do “that our joy may be full.” In order for the Christians’ joy to be complete, the truth must be communicated, and followed.