It was critical that the elect lady and her children abide in love, and in the doctrine of Christ, because many deceivers were out and about who would overthrow the faith of God’s children if at all possible. Having reminded them of the duty of Christian love, the faithful apostle would also warn them against the heretics who sought to creep into the churches, and diminish the person and glory of Christ. Satan wasted no time when the Gospel first began to go forth with power, but immediately began to insinuate his servants into the churches, to sow the seeds of division, and to begin to disseminate false doctrine. Many and varied were his tactics. In the Galatian churches and in others, he sought to dissuade the people from simple faith in Christ by urging them to look to circumcision and personal obedience as a part of their justification. In other places, as we learn from all the apostolic writings, he sent in ungodly men who sought to turn the grace of the Lord Jesus Christ into an excuse for lasciviousness. These were damning and dangerous tactics, but perhaps the most abominable and destructive of all was the one mentioned here by John, those who would attack the very person of our glorious Saviour. So adamantly opposed to these heretics was the apostle John, that he denounces them in this place as “a deceiver and an antichrist.”
Their lie was that Jesus Christ was not come in the flesh. This could refer to one of two heresies, or perhaps could encompass both. There were Gnostics and certain mystical heretics floating about (as there still are today) who denied the true humanity of Christ. They held that His was not a real human body, but only the spiritual appearance of a body. Probably it is for this reason that John laid such great stress in his gospel upon the human characteristics of Christ. He addressed his human emotion and zeal for the glory of God in driving the moneychangers out of the temple (chapter 2). He observed Jesus as a man weary with a long journey (chapter 4). He was a man beset with the unbelief of those around Him, even His own brethren (chapter 7). He could kneel down and doodle in the dirt, like any other man (chapter 8). He wept with much sorrow for the grief and unbelief of others (chapter 11). And then we have John’s vivid account of the sufferings of Christ, where we see Him scourged, nailed to a cross, and then even after His death, blood and water flowing from His pierced side (chapters 18 and 19). Even after His resurrection, John notices the humanity of Christ, in that He could sit and eat with His disciples, even in His glorified state. To deny the humanity of Christ, then, is to be a deceiver and an antichrist.
But it may be also that John is attacking those who denied the pre-existence of the Son of God. There have always been heretics who would argue that Christ had no personal existence before His conception in the womb of Mary. This is an utter denial of the doctrine of the apostles, who always laid great stress upon the wonder of God “sending His Son,” a plain intimation that the Son was with the Father, but was sent by the Father into the world upon a mission of salvation. It also refutes the words heard by John from the very lips of the Saviour Himself, Who prayed to His Father that He might be restored to that glory which He possessed with the Father before the world began (John 17:5). In verse 24 of the same chapter, in the same prayer, Jesus spoke of the love of the Father which He enjoyed before the foundation of the world. To deny, then, that the Son of God existed in glory and power with the Father from the days of old eternity, is to be a deceiver and an antichrist. John would have the elect lady and her children, and in fact all true Christians, to be on guard against such heretics who would pervert the Gospel by assaulting the very person of our dear Saviour.