Charles Spurgeon 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 every spirit that confesses not that Jesus Christ has come in the flesh is not of God: and this is that spirit of antichrist, of which you have heard that it should come; and even now already is it in the world.

That was the form that antichrist took in John's day. It is constantly taking different forms, but it is always anti-Christ—against Christ.

And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.

The devil is up early at his evil work; we may sleep, but he never does.

This is that spirit of antichrist, about which ye have heard that it should come; and even now already is it in the world.

And every spirit that does not confess that Jesus Christ has come in the flesh is not of God: and this is that spirit of antichrist, of which you have heard that it should come; and even now already is it in the world.

So that there were deadly errors very early in the history of the Christian Church. The loving apostle John did not handle them with gloved hands, but he dealt with them honestly and sought to destroy them. We must not wonder if, in our days, the church has many heresies in it; but they are not to be tolerated, but to be cut up, root and branch.