John began the Gospel by declaring the Word was God in the beginning (1:1), but the Word became flesh (1:14). This balances the divinity of Jesus with his real human flesh.
What are the opponents teaching about Jesus? The usual suggestions include Docetism (Jesus only appeared to have real flesh since human flesh is by nature evil), Cerenthus (the earthly Jesus is different than heavenly Jesus), or even a teaching that Jesus was flesh, but his flesh was important for our salvation (Von Wahlde, Epistles of John, 142-43). The problem is the later descriptions of these classic Christological heresies fit what John says. Perhaps it is better to take this false teaching about Jesus John is concerned about as similar to these later rejected Christologies.
For John, the test is not that someone should confess Jesus was really human, but that they confess Jesus Christ, the one who has come in the flesh. In some ways, this is shorthand for “Jesus as presented in my previous book the Gospel of John, especially in the prologue, John 1:1-18.” Obviously John would not refer to his book in this modern form of citation, but if one were to ask John “what do you mean by when you sat Jesus Christ,” he would respond with the content of John 1:1-18.
Based on the content of 1 John, it is likely the opponents were denying Jesus was fully human and also that he was the Jewish messiah (the Christ). Based on John 20:30-31 the purpose of the whole Gospel of John was to convince the reader to believe that Jesus is the Messiah and that by believing in him to have life in his name; that name is Jesus Christ.
To read this as simply a confession that was a human would mean anyone who thought Jesus really existed historically could be speaking the truth and therefore acceptable as a teacher in John’s churches. This is obviously not the case, since someone could announce they believe in the historical Jesus but not that he was the son of God or that he was crucified and raised from the dead.
The spirit which does not confess Jesus Christ came in the flesh is antichrist. This recalls the beginning of this portion of the letter, recalling 2:18. In 4:5 John says these false teachers are speaking from viewpoint of the world and are therefore speaking in the “spirit of falsehood.”
John’s warning is clear, do not believe everyone who claims to be a Spirit led teacher because not everyone is speaking the truth. John’s readers do not need to fear these false teachers since God is greater than anyone in the world.