God’s nature did not change when He became a man in the person of Jesus. First of all, the Bible teaches the doctrine of the Trinity--that the one God exists as three distinct persons: Father, Son, and Holy Spirit. Before the Son became flesh, He was known as “The Word.” John 1:1, 14, “In the beginning was the Word, and the Word was with God, and the Word was God . . . 14 And the Word became flesh, and dwelt among us, and we beheld His glory, glory as of the only begotten from the Father, full of grace and truth.” When the Word became flesh, He was born of the Virgin Mary and named Jesus. So, it was not the Father or the Holy Spirit who became flesh but the Word alone.
Furthermore, the Bible tells us that Jesus has two natures: divine and human. Col. 2:9 says, “For in Him all the fulness of Deity dwells in bodily form.” So the divine nature of God remained unchanged but was united to the human nature in the person of Jesus. Again, Jesus has two natures: divine and human, and the divine nature is not changed at the incarnation.