If God does not become flesh, He remains the Spirit both invisible and intangible to man. Man is a creature of flesh, and man and God belong to two different worlds and are different in nature. The Spirit of God is incompatible with man of flesh, and no relations can be established between them; moreover, man cannot become a spirit. As such, the Spirit of God must become one of the creatures and do His original work. God can both ascend to the highest place and humble Himself by becoming a man of creation, doing work and living among man, but man cannot ascend to the highest place and become a spirit and much less can he descend to the lowest place. Therefore, God must become flesh to carry out His work. … Only the flesh of God incarnate could redeem man through His crucifixion, whereas it was not possible for the Spirit of God to be crucified as a sin offering for man. God could directly become flesh to serve as a sin offering for man, but man could not directly ascend to heaven to take the sin offering that God had prepared for them. As such, God must journey to and fro between heaven and earth, rather than letting man ascend to heaven to take this salvation, for man had fallen and could not ascend to heaven, much less obtain the sin offering. Therefore, it was necessary for Jesus to come among men and personally do the work that simply could not be accomplished by man.

—The Word, Vol. 1. The Appearance and Work of God. The Mystery of the Incarnation (4)

The first stage of God’s work was the leadership of man. It was the start of the battle with Satan, but this battle had yet to officially begin. The official war with Satan began with the first incarnation of God, and it has continued right up until today. The first instance of this war was when God incarnate was nailed to the cross. The crucifixion of God incarnate defeated Satan, and it was the first successful stage in the war. When God incarnate begins to directly work the life of man, this is the official start of the work of regaining man, and because this is the work of changing man’s old disposition, it is the work of doing battle with Satan. The stage of work done by Jehovah in the beginning was merely the leadership of man’s life on earth. It was the beginning of God’s work, and although it had yet to involve any battle, or any major work, it laid the foundation for the work of the battle to come. Later, the second stage of work during the Age of Grace involved changing man’s old disposition, which means that God Himself wrought the life of man. This had to be personally done by God: It required that God personally become flesh, and if He had not become flesh, no one else could have replaced Him in this stage of work, for it represented the work of fighting directly against Satan. If man had done this work on God’s behalf, when man stood before Satan, Satan would not have submitted and it would have been impossible to defeat it. It had to be God incarnate who came to defeat it, for the substance of God incarnate is still God, He is still the life of man, and He is still the Creator; whatever happens, His identity and substance will not change. And so, He put on the flesh and did the work to cause the complete submission of Satan.

—The Word, Vol. 1. The Appearance and Work of God. Restoring the Proper Life of Man and Taking Him to a Wonderful Destination