Agere sequitur esse. This venerable scholastic dictum—i.e., “action follows upon being”—encapsulates the point at issue in Summa Theologiae I, Q. 4–6. What a thing does is a ‘second act’ following upon what a thing is, its ‘first act’—i.e., its act to be that which it is.
Since God, on the other hand, is Ipsum Esse (existence itself), everything He does is one with what, or that, He Is.
Whatever good a person—or any created substance—achieves is resultant upon its first act, its act of being, and can fall short of goodness or perfection. Form and Goodness, while distinct in rational analysis, are in Nature—as in God—not in fact distinct; they are one. Ideally, every thing, by simply being what it is (morphe) achieves its perfection (telos).
God is wholly actual—His Nature is ‘existence’—and created beings—or ‘existents’—participate in His primordial act-of-existing. But only to a certain extent. The being of creatures is analogous: the manner and degree of the being of creatures is prescribed by the delimitation of each thing’s essence. Note well that this entails a real distinction between essence and existence / act-of-being in every creature, whereas in the Creator there is no such distinction: rather, His ‘essence’ is ‘to be / exist’.
Humanity—because it is Fallen and possessing free will—indeed ‘falls short’ of its own perfection, causing an intrinsic lack of full participation in its own essence (human ‘being’ in the strict sense), what humanity is created to be. And even in Nature—composed of creatures subject to generation and corruption—we find beings which fall short of their true essence, especially by the phenomenon of death or in the process of decay.
But since in God there is no distinction between His attributes and His existence—and since we are certain of His existence by the Five Ways—we can be likewise certain of His goodness and His perfection.