From my recent paper (“St. Paul on Jesus Christ, ‘Our Great God and Savior'”):
All creatures change, but God is immutable: “I the LORD do not change” (Malachi 3:6; qtd. in ST I, q. 9, art. 1). St. Paul assigns to Christ the perfection of immutability, meaning that Christ is God incarnate: “Jesus Christ is the same yesterday and today and forever” (Hebrews 13:8; qtd. in ST III, q. 16, art. 9). As God, He has not changed, is not changing, and will never change, unlike all of us, His servants.