Agere Sequitur Esse

Immutability of God in His Activity

The fact that God produces things by His will clearly shows that He can produce new things without any change in Himself. The difference between a natural agent and a voluntary agent is this: a natural agent acts consistently in the same manner as long as it is in the same condition. Such as it is, thus does it act. But a voluntary agent acts as he wills. Accordingly it may well be that, without any change in himself, he wishes to act now and not previously. For there is nothing to prevent a person from willing to perform an action later, even though he is not doing it now; and this without any change in himself. Thus it can happen, without any change in God, that God, although He is eternal, did not bring things into existence from eternity.

Reference

St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Immutability of God in His Activity, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947