‘By this you know the Spirit of God. Every spirit which confesses that Jesus Christ is come in the flesh is of God, and every spirit which does not confess Jesus is not of God, and this is that of the antichrist, of which you have heard that it is coming. And now it is in the world already.'
So the spirits of the prophets had to be tested against revealed truth which would determine whether their spirits were inspired by the Holy Spirit. If the prophecy was of new, previously untaught ideas it was probably false. But it could easily be tested out. The Spirit of God at work through the spirits of the prophets (‘every spirit') could be tested in this way. If they were true their Spirit-inspired spirits would testify that Jesus, came as a human being in the flesh, but was equally the Christ, the One Who was uniquely ‘the Son' (1 John 2:22-23). For that was the revealed truth as proclaimed by the Apostles, and prepared for in the Old Testament.
On the other hand those whose spirits do not prophesy of Jesus at all (but only speak of ‘the Christ') are not of God, for what they teach is false. Rather they are that which is of antichrist, setting up false Christs instead of the true. The believers in the churches had heard that such were coming. Well, here they were, already in the world. Let them then listen carefully to what was taught before accepting a prophet.
The centrality of Jesus Christ to the Christian faith comes out here. Their view of Jesus Christ was of central importance. Christianity was Christ. All hinged on Him. To be wrong about Him was not to be a Christian at all. Teachers were thus to be tested by whether they proclaimed Jesus Christ, in His life, death and resurrection, as true man and true God.