Immutability means that God is unchangeably true to Himself and thus to His purposes and promises (Mal. 3:6). He is sovereign and is not subject to external agents or forces. For that reason, His character does not and cannot change. This is the foundation for all His external works of creation, providence, and grace. It is the basis for our faith and assurance (Heb. 6:13–20).
Some have suggested that the incarnation was something new for God. It involved the Son, one member of the Trinity, taking to Himself in permanent, everlasting personal union a human nature, which in that act was constituted His human nature. From the historical angle, this was a real event in our own world that happened just over two thousand years ago. Before then, it had not occurred; afterward, it had.
But it was God’s primal decision to become incarnate in Jesus Christ from then and forever, as the foundation of our own election and union with Him (Eph. 1:4). Moreover, in the incarnation, God Himself did not change. The Son did not become man in the sense of changing into man. He did not enhance Himself or add to what He always was and is. That would not be incarnation but metamorphosis. Rather, the Son by taking human nature into permanent union so that it became His human nature (Phil. 2:6–7) became flesh in such a way that He continued, as the Son or Logos, to be the subject of all the experiences of Jesus of Nazareth, while remaining who He ever was and is (John 1:1–4, 14–18; Heb. 13:8), experiencing them as man. From that, the processes of human life in this world, including suffering, death, burial, and resurrection, are known to God from a human perspective. The immutability of God secures this.
implications and entailments
Immutability (together with simplicity and impassibility) is foundational to the whole of theology. It provides the ground for the work of redemption, since God is relentlessly true to His covenant and its promises. It is a vital bulwark for our assurance (Heb. 6:13–20), for “the steadfast love of the Lord is from everlasting to everlasting on those who fear him” (Ps. 103:17).