By this you know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, (1 John 4:2).
In the day when this book was written, the biggest threat against the church was false doctrine, and the worst of the false doctrines was Agnosticism, which was the belief that Jesus did not come in the flesh, but was a spirit who only looked human. Why is this belief so bad? If Jesus did not come in the flesh, then He could not have paid for our sins with His blood. He could not have been a model for how a human being should act with the Holy Spirit of God. Everything He taught would have been a misrepresentation of who He was.
Yes, He was indeed 100% man and 100% God. People try to argue this point today, wanting to make of Jesus a being that is untouchable by us humans. The Bible says that He put aside His Deity in order to become 100% man. He had to be touchable or He was of no value to us. We had to know that we had a God who came to our level and lived among us as one of us. That fact alone lifts us to His level. Those false doctrines claimed that the flesh was evil and the spirit was good. We know today that it is an honor to wear this flesh, that Jesus did it too, and we are seated with Him on His throne in heaven. If He didn’t come in the flesh, He couldn’t have lifted our heads to be level with His.