1 John 4:1-3
On Denying the Incarnation
4 Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. 2 This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, 3 but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world.
John summarizes his teaching about false teachers today by telling the believers to "not believe every spirit", but test the spirits to see whether they are from God or not. A person's teaching flows from their spirit within them. Notice the spiritual reality of what is taught regarding spiritual matters. If someone purports to impart a spiritual message it is via one of two spirits. A spirit of the world, or from the Spirit of God. And John gives a litmus test for which spirit it is. He says you can recognize the Spirit of God by how they teach about Jesus Christ.
Further he adds if a spirit does not acknowledge that "Jesus has come in the flesh" it is not of God. Why is that so important? One of the earliest heresies is called "docetism", from the Greek word "dokeo", which means to "seem or appear". Synonyms would be "phantom" or "illusory". This came from a false belief that God could not enter a human body, so Jesus was not really human.
So what's the big deal about that? The very big deal is that if Jesus isn't really human, he did not come in the flesh. If he didn't really come as a human being, he really didn't die on the cross. And if he really didn't die on the cross, then how are our sins forgiven. How can Jesus be the perfect substitute for our sins, if he didn't take God's punishment on the cross? So you can see why this is such a big heresy. It takes the very heart out of Christianity. This was universally condemned at the Council of Nicea in 325 AD, which produced our Nicene Creed.
Here is what the Second Article about Jesus says about what we believe in...
"And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all worlds, God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father; by whom all things were made; who for us men, and for our salvation, came down from heaven, and was incarnate by the Holy Ghost of the Virgin Mary, and was made man, and was crucified also for us under Pontius Pilate; He suffered and was buried; and the third day He rose again according to the Scriptures; and ascended into heaven, and sitteth on the right hand of the Father; and He shall come again with glory to judge the quick and the dead; whose kingdom shall have no end."
Notice it is says "was made man" Jesus' incarnation is one of the precious doctrines we hold as Christians. In it we realize God is with us. As we head into Christmas it is the center of what it meant for him to come into the world as a baby. He didn't just seem to be a baby, he was one. One of the ways we are comforted in our suffering is to realize that Jesus suffered in a human body. He knows what we are going through because he became one of us.