(CG 1/026). That God is not the formal or abstract being of all things (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

AS all perfection and nobility is in a thing inasmuch as the thing is, so every defect is in a thing inasmuch as the thing in some manner is not. As then God has being in its totality, so not-being is totally removed from Him, because the measure in which a thing has being is the measure of its removal from not-being. Therefore all defect is absent from God: He is therefore universal perfection.

2. Everything imperfect must proceed from something perfect: therefore the First Being must be most perfect.

3. Everything is perfect inasmuch as it is in actuality; imperfect, inasmuch as it is in potentiality, with privation of actuality. That then which is nowise in potentiality, but is pure actuality, must be most perfect; and such is God.

4. Nothing acts except inasmuch as it is in actuality: action therefore follows the measure of actuality in the agent. It is impossible therefore for any effect that is brought into being by action to be of a nobler actuality than is the actuality of the agent. It is possible though for the actuality of the effect to be less perfect than the actuality of the acting cause, inasmuch as action may be weakened on the part of the object to which it is terminated, or upon which it is spent. Now in the category of efficient causation everything is reducible ultimately to one cause, which is God, of whom are all things. Everything therefore that actually is in any other thing must be found in God much more eminently than in the thing itself; God then is most perfect.

Hence the answer given to Moses by the Lord, when he sought to see the divine face or glory: I will show you all good (Exod. xxxiii, 19).