The fulfillment of things spoken by God shows that he does not change
4. But then, dear friends, let us look at the past: and there we shall gather some proofs of God’s immutable nature. “Hath he spoken, and hath he not done it? Hath he sworn, and hath it not come to pass?” Can it not be said of Jehovah, He hath done all his will, and he hath accomplished all his purpose?” Turn ye to Philistia; ask where she is. God said, Howl Ashdod, and ye gates of Gaza, for ye shall fall,” and where are they? Where is Edom? Ask Petra and its ruined walls. Will they not echo back the truth that God hath said, “Edom shall be a prey, and shall be destroyed?” Where is Babel, and where Nineveh? Where Moab and where Ammon? Where are the nations God hath said he would destroy? Hath he not uprooted them and cast out the remembrance of them from the earth? And hath God cast off his people? Hath he once been unmindful of his promise? Hath he once broken his oath and covenant, or once departed from his plan? Ah! no. Point to one instance in history where God has changed! Ye cannot sirs; for throughout all history there stands the fact, that God has been immutable in his purposes. Methinks I hear some one say, “I can remember one passage in Scripture where God changed!” And so did I think once. The case I mean, is that of the death of Hezekiah. Isaiah came in and said, “Hezekiah, you must die, your disease is incurable, set your house in order.” He turned his face to the wall and began to pray; and before Isaiah was in the outer court, he was told to go back and say, “Thou shalt live fifteen years more.” You may think that proves that God changes; but really I cannot see in it the slightest proof in the world. How do you know that God did not know that? Oh! But God did know it- he knew that Hezekiah would live. Then he did not change, for if he knew that, how could he change? That is what I want to know. But do you know one little thing?-that Hezekiah’s son Manasseh, was not born at that time, and that had Hezekiah died, there would have been no Manasseh, and no Josiah, and no Christ, because Christ came from that very line. You will find that Manasseh was twelve years old when his father died; so that he must have been born three years after this. And do you not believe that God decreed the birth of Manasseh, and foreknew it? Certainly. Then he decreed that Isaiah should go and tell Hezekiah that his disease was incurable, and then say also in the same breath, “But I will cure it, and thou shalt live.” He said that to stir up Hezekiah to prayer. He spoke, in the first place as a man. “According to all human probability your disease is incurable, and you must die.” Then he waited till Hezekiah prayed- then came a little “but” at the end of the sentence. Isaiah had not finished the sentence. He said, “You must put your house in order for there is no human cure- but” (and then he walked out. Hezekiah prayed a little, and then he came in again, and said) “But I will heal thee.” Where is there any contradiction there, except in the brain of those who fight against the Lord, and wish to make him a changeable being.
Charles H. Spurgeon- The Immutability of God- A sermon delivered on Sabbath morning, Jan 7th, 1855