Thomas Aquinas, Compendium of Theology

All this brings to light the different relationship that God and creatures have to goodness. We may examine this difference from the standpoint of the two kinds of goodness discerned in creatures. Since the good has the nature of perfection and of end, the twofold perfection and end of the creature disclose its twofold goodness. A certain perfection is observed in the creature inasmuch as it persists in its nature. This perfection is the end of its generation or formation. The creature has a further perfection which it reaches by its motion or activity. This perfection is the end of its movement or operation.

In both kinds of perfection the creature falls short of the divine goodness. The form and existence of a thing are its good and perfection when considered from the standpoint of the thing's nature. But a composite substance is neither its own form nor its own existence; and a simple substance, although it is its own form, is not its own existence. God, however, is His own essence and His own existence, as was shown above.

Likewise, all creatures receive their perfect goodness from an end extrinsic to them. For the perfection of goodness consists in attainment of the ultimate end. But the ultimate end of any creature is outside the creature. This end is the divine goodness, which is not ordained to any ulterior end. Consequently God is His own goodness in every way and is essentially good. This cannot be said of simple creatures, because they are not their own existence, and also because they are ordained to something external as to their ultimate end. As for composite substances, clearly they are not their own goodness in any way. Hence God alone is His own goodness, and He alone is essentially good. All other beings are said to be good according as they participate, to some extent, in Him.