Immutability is the inability to mutate, or change. God does not change. He cannot become a better God, nor can He become worse. In order to be God, He must be perfect. Something that is perfect, by definition, cannot worsen or improve; it cannot grow, deteriorate, or change. God's immutability is central to His being. When Moses asked what to call Him, God replied with "I AM" (Exodus 3:14). Jesus used this same phrase several times to identify Himself as God. For example, in John 8:58, Jesus said, "Truly, truly, I say to you, before Abraham was, I am." The people picked up stones to throw at Him because they understood He was claiming to be God. God simply is and He cannot change. So how is it that Jesus could be both God and man? How can God take on human flesh? This is a question that has been batted around for centuries.
Can the incarnation be reconciled with the immutability of God?
First, it's important to grasp the divinity of Jesus. He claimed to be God repeatedly when He was on earth. The disciples and others eventually came to a convincing belief in Jesus' divinity and wrote about it (for example, see John 1:1–14; Philippians 2:5–11; Colossians 1:15–20). Of course, Jesus was fully human as well. He grew from a child into a man (Luke 2:52) and was subject to human weakness such as experiencing fatigue (John 4:6). He also experienced death. Would God put up with that? Probably not a god we would create, but the true God did because it followed His sovereign plan to set us free from sin and death. Jesus' resurrection from the dead proves that He is God and that everything He said and taught is true.
Was Jesus a brilliant idea God had one day, so He set about finding a virgin and lining up His birth in Bethlehem and all the other answers to centuries of prophesies about Him? No. Jesus was present at the beginning of creation, as were the Holy Spirit and the Father (John 17:5; Acts 3:15; Colossians 1:16–17).
God is one God, in three distinct persons. The church used to call this the "mystery of the Trinity." There is mystery in the reality of the Trinity. However, there is no question that the Trinity has always been. It is not a newer iteration of God; God has always existed as three persons.
God is unchanging in His nature, His character, His purposes, and His plan. Coming to earth as a man, as Messiah, as Jesus, fully man, was always the plan and a portion of the purpose of Jesus' personhood within the Trinity. When Jesus took on human flesh, He also took on a human nature. His divinity did not blend with His humanity; rather the two natures coexist in what is called the hypostatic union. As God, Jesus is unchanging. In His humanity, He could experience change. For example, He "increased in wisdom and in stature and in favor with God and man" (Luke 2:52). Great theological minds are unable to fully explain the mystery of the incarnation. But the biblical evidence is clear—Jesus is both God and man, and God is unchanging.
Great truths about the reality of our salvation in Jesus Christ flow from this mystery of His incarnation. First Timothy 2:5 tells us, "For there is one God, and there is one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all, which is the testimony given at the proper time." As God, Jesus perfectly represents God to us and He is able to pay the infinite debt of our sin. As sinless human, Jesus can empathize with our weakness and serve as the propitiation for our sins and advocate for us before God (Hebrews 4:15; 1 John 2:1–2).
Who is Jesus Christ?
What is the theological concept of the hypostatic union?
What is the incarnation of Christ and why is the incarnation important?
Is Jesus a myth?
Is Jesus Christ God?
Truth about Jesus Christ