The sixth/seventh century churchman Isidore of Seville authored, among other things, an early collection of Christian Sententiae, in three books. Most of the opinions in it are drawn from Augustine and Gregory the Great.
The first book begins with God. Its first chapter has the title Quod Deus summus et incommutabilis sit (“That God is the highest and immutable”; or, “That the highest God is also immutable”–which is to be preferred will emerge as we read through the chapter). We will look at them one at a time, and maybe we can all learn something, hey? So, Sententiae 1.1.1:
1. Summum bonum Deus est, quia incommutabilis est, et corrumpi omnino non potest. Creatura vero bonum, sed non summum est, quia mutabilis est; et dum sit quidem bonum non tamen esse potest et summum.
1. God is the highest good, because he is immutable, and can in no way be marred [i.e., changed for the worse]. Creation, on the other hand, is good, but it is not the highest [good], because it is mutable; and, although it is indeed good, it nevertheless cannot be also the highest [good].
Isidore begins with a question that had greatly exercised ancient philosophers, viz., “What is the highest good (summum bonum)?” and declares that it is God himself. The reason, he says, is that God is unchangeable. Thus it is implied that God is perfect, for any change in him would be a change for the worse. That is why Isidore uses the verb corrumpi rather than something more neutral, such as mutari: change in God would be a corruption of his perfection. Because of God’s unchangeability linked with his absolute perfection in the two clauses dependent on quia, we can conclude that God is the highest good; he has goodness in immutable fullness.
The second sentence introduces the important distinction between creator and creation. The world God has made is good; but it cannot be the highest good, because it, unlike God, is changeable. If that is the case, it can be changed for the worse, and thus its goodness is corruptible, or it can be changed for the better, and thus it lacks maximal goodness. In either instance, its goodness and perfection are not absolute, and so it cannot be construed as the summum bonum. Envisioning a hierarchy of goods allows one to make such distinctions between what is good and what is the highest good. Creation, then, has real (thought mutable) goodness bestowed upon it by God, but in the nature of the case the immutable one who bestows it is still greater.