John Calvin Commentary 1 John 4:3

"and every spirit that confesseth not Jesus is not of God: and this is the [spirit] of the antichrist, whereof ye have heard that it cometh; and now it is in the world already." — 1 John 4:3 (ASV)

And this is that spirit of Antichrist. The Apostle added this to make the deceptions that lead us away from Christ more detestable. We have already said that the teaching concerning the kingdom of Antichrist was well known, so that the faithful had been warned about the future scattering of the Church, in order that they might exercise vigilance. Therefore, they justly dreaded the name as something vile and ominous. The Apostle now says that all those who depreciated Christ were members of that kingdom.

And he says that the spirit of antichrist would come, and that it was already in the world, but in a different sense. He means that it was already in the world because it secretly carried on its iniquity. However, since the truth of God had not yet been subverted by false and spurious dogmas, since superstition had not yet prevailed in corrupting the worship of God, since the world had not yet treacherously departed from Christ, and since tyranny, opposed to the kingdom of Christ, had not yet openly exalted itself, he therefore says that it would come.