Quoted by Confessing Baptist:
But God is perpetually the same. He is not composed of any substance or material, but is spirit—pure, essential, and ethereal spirit—and therefore he is immutable. He remains everlastingly the same. There are no furrows on his eternal brow. No age hath palsied him; no years have marked him with the mementoes of their flight; he sees ages pass, but with him it is ever now. He is the great I AM—the Great Unchangeable.
Mark you, his essence did not undergo a change when it became united with the manhood. When Christ in past years did gird himself with mortal clay, the essence of his divinity was not changed; flesh did not become God, nor did God become flesh by a real actual change of nature; the two were united in hypostatical union, but the Godhead was still the same. It was the same when he was a babe in the manger, as it was when he stretched the curtains of heaven; it was the same God that hung upon the cross, and whose blood flowed down in a purple river, the self-same God that holds the world upon his everlasting shoulders, and bears in his hands the keys of death and hell.
He never has been changed in his essence, not even by his incarnation; he remains everlastingly, eternally, the one unchanging God, the Father of lights, with whom there is no variableness, neither the shadow of a change.