We cannot believe all who claim to be full of the Spirit and speak for God. We must "test the spirits" to see if they are indeed from God.
John shows his continued concern for false teachers and the "antichrists" who oppose the truth. John's test to see if a teaching is of the truth is focused on Christ: every spirit that confesses that Jesus has come in the flesh (that is, that Jesus was fully human, existing as a human body) is of God. No doubt the false teachers were denying this in some way, even though it is difficult to know precisely what they taught.
4Beloved, do not believe every spirit, but test the spirits to see whether they are from God; for many false prophets have gone out into the world. 2By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, 3and every spirit that does not confess Jesus* is not from God. And this is the spirit of the antichrist, of which you have heard that it is coming; and now it is already in the world. 4Little children, you are from God, and have conquered them; for the one who is in you is greater than the one who is in the world.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org