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 is the Messiah whose life, death, and resurrection are God's saving act for humanity 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.