If God were not a Spirit, he were not immutable and unchangeable. His immutability depends upon his simplicity. He is unchangeable in his essence, because he is a pure and unmixed spiritual Being, Whatsoever is compounded of parts may be divided into those parts, and resolved into those distinct parts which make up and constitute the nature. Whatsoever is compounded is changeable in its own nature, though it should never be changed.

Charnock, Stephen. The Existence and Attributes of God . Kindle Edition.