John opens this letter with a firm declaration of his authority as an eyewitness of the incarnation (as Paul often did). Despite the fact that some denied the truth of the incarnation (as we discover later), John asserts that he and the apostles know the reality of the resurrection through firsthand experience.
John’s audience had experienced a break in fellowship when the incarnation-deniers left. But John tells them that what they need is fellowship with him, because he is in fellowship with the Father and Son. John is saying to this church, “Stay with us. Do not follow those who left.” As John will make clear later, break in fellowship is not to be desired. If such a rupture happens, however, John counsels his audience to stay with those connected to the apostolic witness, which is connected to God.
Notice John’s two stated pastoral purposes. As just noted, his first purpose is that his readers remain in fellowship with orthodox believers and thus in fellowship with God. In other words, John is pursuing their perseverance. It is true that God preserves his people, but one way he does so is through the teaching, rebuking, and correcting of pastors and other believers. If we turn a deaf ear to such instruction, we ought not expect to persevere (Prov. 19:27). Secondly, John writes to ensure the joy of his people. This also shows us the heart of a true pastor, yearning for and laboring for the perseverance of his people. John “has the heart of a pastor which cannot be completely happy so long as some of those for whom he feels responsible are not experiencing the full blessings of the gospel.”*
*I. Howard Marshall, The Epistles of John, NICNT (Grand Rapids, MI: Eerdmans, 1978), 105.