The immutability of God is his changelessness with respect to his person, attributes and purposes; his incapability of growth or decay in any respect.
God has never grown or diminished; he has never learned or forgotten anything; he has not changed his mind about anything, nor lost any of his power. He is neither more nor less merciful, loving, wise, truthful, etc., than he ever was.
God is unchangeable in his:
(1) Purpose: Ezekiel 24:14—I am the LORD. I have spoken; it shall come to pass; I will do it. I will not go back; I will not relent.
(2) Decree: Psalm 33:11—The plans of the Lord stand forever.
Isaiah 46:11— What I have said, that will I bring about; what I have planned, that will I do.
(3) Promises: Romans 11:29—God’s gifts and his call are irrevocable.
(4) Person: Malachi 3:6—I the Lord do not change.
Hebrews 1:11–12—They will perish, but you remain; they will all wear out like a garment. You will roll them up like a robe; like a garment they will be changed. But you remain the same, and your years will never end.
(5) Character: James 1:17—The Father does not change like shifting shadows.
Stand Firm! 