John Calvin Commentary 1 John 4:2

"Hereby know ye the Spirit of God: every spirit that confesseth that Jesus Christ is come in the flesh is of God:" — 1 John 4:2 (ASV)

By this, or by this, you know he lays down a special mark by which they might more easily distinguish between true and false prophets. Yet he only repeats here what we have met with before: that as Christ is the object at which faith aims, so he is the stone at which all heretics stumble. Therefore, as long as we abide in Christ, there is safety; but when we depart from him, faith is lost, and all truth is rendered void.

But let us consider what this confession includes. For when the Apostle says that Christ came, from this we conclude that he was with the Father before, by which his eternal divinity is proved.

By saying that he came in the flesh, he means that by putting on flesh, he became a real man, of the same nature with us, so that he might become our brother—except that he was free from every sin and corruption.

And lastly, by saying that he came, the cause of his coming must be noted, for he was not sent by the Father for nothing. Therefore, on this depend the office and merits of Christ.

Therefore, just as the ancient heretics departed from the faith—in one instance by denying the divine, and in another by denying the human nature of Christ—so do the Papists at this day. Though they confess Christ to be God and man, they by no means retain the confession which the Apostle requires, because they rob Christ of his own merit. For where free will, merits of works, fictitious modes of worship, satisfactions, and the advocacy of saints are set up, how very little remains for Christ!

The Apostle, therefore, meant this: that since the knowledge of Christ includes the sum and substance of the doctrine respecting true religion, our eyes ought to be directed to and fixed on him, so that we may not be deceived. And doubtless, Christ is the end of the law and the prophets; nor do we learn anything else from the gospel but his power and grace.