City of God

This may be enough to prevent any one from supposing, when we speak of the apostate angels, that they could have another nature, derived, as it were, from some different origin, and not from God. From the great impiety of this error we shall disentangle ourselves the more readily and easily, the more distinctly we understand that which God spoke by the angel when He sent Moses to the children of Israel: “I am that I am.” For since God is the supreme existence, that is to say, supremely is, and is therefore unchangeable, the things that He made He empowered to be, but not to be supremely like Himself.

To some He communicated a more ample, to others a more limited existence, and thus arranged the natures of beings in ranks. For as from sapere comes sapientia, so from esse comes essentia—a new word indeed, which the old Latin writers did not use, but which is naturalized in our day, that our language may not want an equivalent for the Greek οὐσία. For this is expressed word for word by essentia. Consequently, to that nature which supremely is, and which created all else that exists, no nature is contrary save that which does not exist. For nonentity is the contrary of that which is. And thus there is no being contrary to God, the Supreme Being, and Author of all beings whatsoever.