Divine immutability is the entire freedom of God from the vicissitudes of change and contingency. Rightly understood it does not constitute a restriction of His personal life and operations, but characterizes their transcendent perfection and fulness. The very infinitude from which divine immutability is deduced forbids all external limitation.
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Divine immutability involves (a) that no change, whether of accretion or subtraction, of growth or corruption, or of alteration in manner of existence and internal relation, can take place in the divine essence or attributes; (b) that the operations of God, which may not be evaporated of reality by the manner in which we hold this truth, are eternal in themselves, and involve no transition or mutation in God. They may have changes for their effects, but an eternal operation cannot itself be changed; (c) that the changes which occur in the relations between God and creaturely things and events are not changes in God or in His actions, although we do not err necessarily in symbolically describing His actions in temporal terms – terms which, strictly speaking, apply only to their effects in history.
The truth of divine immutability, however abstract it may be in its theological explication, has practical value: (a) It affords a guarantee that science will never be stultified in assuming that the fundamental principles by which natural phenomena are governed will never cease to control them; (b) It assures us of the inviolabilty of the laws of the moral order, and of the principles that lie behind the mystery of the Cross and the future dispensation of mercy and justice; (c) It enables us to rely with confidence upon the promises of God, and upon the eternal purposes which they reveal.
The Being and Attributes of God, Francis J. Hall, D.D., New York:Longmans, 1918, pp. 256-260.