There is nothing more crucial than God’s Immutability, the truth that God’s essence, promises, eternal plans, and character cannot improve or change (Malachi 3:6). If God’s essence can improve, then He wasn’t perfect to begin with. If His essence can change then God cannot guarantee that He will remain faithful to truth without turning to falsehood. This is crucial, because if God is changeable then maybe He’ll decide to change the way of salvation, redefine Good, or make Heaven into Hell. If God is changeable, there’s no telling what to expect when we pass into the next life. But people have wondered, didn’t God become man? Didn’t that involve a change in His essence?
The incarnation did involve a change to the Second Person of the Godhead but not a change in His essence, promises, eternal plans, or character. The Son added to His divine nature a human nature. The divine nature itself didn’t change; rather the divine nature eternally coexists with the human. This is a genuine change, but I think it is quite obvious that coexisting with a human nature doesn’t demand that there be a change of essence in the divine.
The incarnation actually shows how changeless God’s character really is! It shows that the Son’s love, wisdom, and grace didn’t change in the thousands of years since the plan of salvation was devised. God planned salvation from the beginning (Eph. 1:4), and He faithfully carried out that plan through the millennia, culminating in the Incarnation. God’s grace is relentless throughout the eons. His kindness endures forever. It is not fickle. It does not cool off, fade away, or go through ups and downs. His love is indomitable, and in Christ it reaches even me. “Yesterday, today, forever Jesus is the same; all may change but Jesus never; Glory to His name!”
See Hebrews 1:10-12.