1 John 4:2-3 - By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God; and every spirit that does not confess Jesus is not from God; this is the spirit of the antichrist, of which you have heard that it is coming, and now it is already in the world.
John was confronting teaching that Jesus was either not fully God or not fully man. Some claimed that both coexisted, but that the divine left him just before his suffering on the cross. John minces no words – if anyone does not teach Jesus as fully human yet fully divine then that person’s entire teaching is to be rejected. And John only gives two categories for teachers, either teaching from the spirit of God or from the spirit of the antichrist (a class of people against Christ, not the larger than life end of time figure).
The doctrine of Jesus as fully God and fully man is powerful, even if it is supernatural. If he was not a man, then all his miracles, his teaching, and his agony on the cross were only an illusion with no real suffering, no real shedding of blood, and no real atonement for the sins of the world. If Jesus was not fully God then he was not only a liar in His teaching, his death would lack the divine power to justify us before a righteous God. That the incarnation of Jesus produced human nature in permanent union with the divine is an imperative belief for true Christians. Any religion teaching otherwise is a false cultic group to be avoided.
It would be easy to separate out false religious teachers of our day if this were the only category of dangerous heresy, but even the most charismatic, engaging, and eloquent teacher may have one thing so outrageous in his theology that all his teaching should be ignored and categorized with pagan ideas. Even if there are nuggets of truth contain therein, the risk of such teaching makes it too dangerous to consider. There are many things believers can agree to disagree on – infant baptism, juice or wine, age of accountability, etc. – but doctrinal beliefs foundational to the faith must be standard. And though the means of delivering the message to the society can and should change, the true message can never be watered down or changed to fit society’s whims if it is to be of God.