As I have indicated in earlier posts, the doctrine of divine simplicity is absolutely central to classical theism. To say that God is simple is to say that He is in no way composed of parts – neither material parts, nor metaphysical parts like form and matter, substance and accidents, or essence and existence. Divine simplicity is affirmed by such Christian, Jewish, and Muslim thinkers as Athanasius, Augustine, Anselm, Aquinas, Maimonides, Avicenna, and Averroes. It is central to the theology of pagan thinkers like Plotinus. It is the
de fide teaching of the Catholic Church, affirmed at the fourth Lateran council and the first Vatican council, and the denial of which amounts to heresy.
The doctrine of divine simplicity has a number of crucial implications, which are, accordingly, also essential to classical theism. It entails that God is
immutable or changeless, and therefore that He is
impassible – that is, that He cannot be affected by anything in the created order. It entails that he is
eternal in the sense of being altogether outside of time and space. It entails that He does not “have” existence, or an essence, or His various attributes but rather
is identical to His existence, His nature and His attributes: He
is His existence which
is His essence which
is His power which
is His knowledge which
is His goodness.
Source:
Edward Feser: Classical theism