Nicene and Post-Nicene Fathers, Vol. IV:
Writings in Connection with the Manichæan Controversy.: Chapter 19

Chapter 19.—To Have True Existence is an Exclusive Prerogative of God.

Magnificently and divinely, therefore, our God said to his servant:  "I am that I am," p. 355 and "Thou shalt say to the children of Israel, He who is sent me to you." 1090   For He truly is because He is unchangeable.  For every change makes what was not, to be:  therefore He truly is, who is unchangeable; but all other things that were made by Him have received being from Him each in its own measure.  To Him who is highest, therefore nothing can be contrary, save what is not; and consequently as from Him everything that is good has its being, so from Him is everything that by nature exists; since everything that exists by nature is good.  Thus every nature is good, and everything good is from God; therefore every nature is from God.

Footnotes

Ex. iii. 14.