"The Son of God, the second Person of the Trinity, being very and eternal God, of one substance and equal with the Father, did, when the fulness of time was come, take upon Him man's nature, with all the essential properties, and common infirmities thereof, yet without sin; being conceived by the power of the Holy Spirit, in the womb of the virgin Mary, of her substance. So that two whole, perfect, and distinct natures; the Godhead, and the manhood, were inseparably joined together in one person, without conversion ('change') composition, or confusion. Which person is very God and very man, yet one Christ, the only Mediator between God and man." The Westminster Confession of Faith, Chapter VIII, Article II
Bible References: John 1:1, 14; I John 5:20, Philippians 2:6, Galatians 4:4, Hebrews 2:14-17, 4:15; Luke 1:27-35; Colossians 2:9, Romans 9:5, I Peter 3:18, I Timothy 3:16, Romans 1:3-4; I Timothy 2:5
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This biblical teaching of the Incarnation, includes no suggestion of any change necessary within the Godhead, to provide mankind a Savior; the God/Man, Jesus the Christ, who alone could reconcile sinners with their Creator.
God the Son was able and willing to assume the same flesh and blood of His creatures, without relinquishing His divinity and attributes, in order to achieve their salvation. Hebrews 2:10-14 In fact just the opposite to changing His essence, it is because of the divine nature and power of God in which Jesus was conceived, that He was equipped to function fully both as the Son of God and the Son of Man in this world. I John 5:20
Why or what do you think God had to change, in order to fulfill what He had decreed and purposed to do, before the foundation of the world?