1 John 4:2-3

2By this you know and recognize the Spirit of God: every spirit that acknowledges and confesses [the fact] that Jesus Christ has [actually] come in the flesh [as a man] is from God [God is its source];

3and every spirit that does not confess Jesus [acknowledging that He has come in the flesh, but would deny any of the Son’s true nature] is not of God; this is the spirit of the antichrist, which you have heard is coming, and is now already in the world.