Objection 1. It seems that no name can be applied to God substantially. For Damascene says (De Fide Orth. i, 9): "Everything said of God signifies not His substance, but rather shows forth what He is not; or expresses some relation, or something following from His nature or operation."
Objection 2. Further, Dionysius says (Div. Nom. i): "You will find a chorus of holy doctors addressed to the end of distinguishing clearly and praiseworthily the divine processions in the denomination of God." Thus the names applied by the holy doctors in praising God are distinguished according to the divine processions themselves. But what expresses the procession of anything, does not signify its essence. Therefore the names applied to God are not said of Him substantially.
Objection 3. Further, a thing is named by us according as we understand it. But God is not understood by us in this life in His substance. Therefore neither is any name we can use applied substantially to God.
On the contrary, Augustine says (De Trin. vi): "The being of God is the being strong, or the being wise, or whatever else we may say of that simplicity whereby His substance is signified." Therefore all names of this kind signify the divine substance.
I answer that, Negative names applied to God, or signifying His relation to creatures manifestly do not at all signify His substance, but rather express the distance of the creature from Him, or His relation to something else, or rather, the relation of creatures to Himself.
But as regards absolute and affirmative names of God, as "good," "wise," and the like, various and many opinions have been given. For some have said that all such names, although they are applied to God affirmatively, nevertheless have been brought into use more to express some remotion from God, rather than to express anything that exists positively in Him. Hence they assert that when we say that God lives, we mean that God is not like an inanimate thing; and the same in like manner applies to other names; and this was taught by Rabbi Moses. Others say that these names applied to God signify His relationship towards creatures: thus in the words, "God is good," we mean, God is the cause of goodness in things; and the same rule applies to other names.
Both of these opinions, however, seem to be untrue for three reasons.
First because in neither of them can a reason be assigned why some names more than others are applied to God. For He is assuredly the cause of bodies in the same way as He is the cause of good things; therefore if the words "God is good," signified no more than, "God is the cause of good things," it might in like manner be said that God is a body, inasmuch as He is the cause of bodies. So also to say that He is a body implies that He is not a mere potentiality, as is primary matter.
Secondly, because it would follow that all names applied to God would be said of Him by way of being taken in a secondary sense, as healthy is secondarily said of medicine, forasmuch as it signifies only the cause of the health in the animal which primarily is called healthy.
Thirdly, because this is against the intention of those who speak of God. For in saying that God lives, they assuredly mean more than to say the He is the cause of our life, or that He differs from inanimate bodies.
Therefore we must hold a different doctrine--viz. that these names signify the divine substance, and are predicated substantially of God, although they fall short of a full representation of Him. Which is proved thus. For these names express God, so far as our intellects know Him. Now since our intellect knows God from creatures, it knows Him as far as creatures represent Him. Now it is shown above (Question 4, Article 2) that God prepossesses in Himself all the perfections of creatures, being Himself simply and universally perfect. Hence every creature represents Him, and is like Him so far as it possesses some perfection; yet it represents Him not as something of the same species or genus, but as the excelling principle of whose form the effects fall short, although they derive some kind of likeness thereto, even as the forms of inferior bodies represent the power of the sun. This was explained above (Question 4, Article 3), in treating of the divine perfection. Therefore the aforesaid names signify the divine substance, but in an imperfect manner, even as creatures represent it imperfectly. So when we say, "God is good," the meaning is not, "God is the cause of goodness," or "God is not evil"; but the meaning is, "Whatever good we attribute to creatures, pre-exists in God," and in a more excellent and higher way. Hence it does not follow that God is good, because He causes goodness; but rather, on the contrary, He causes goodness in things because He is good; according to what Augustine says (De Doctr. Christ. i, 32), "Because He is good, we are."
Reply to Objection 1. Damascene says that these names do not signify what God is, forasmuch as by none of these names is perfectly expressed what He is; but each one signifies Him in an imperfect manner, even as creatures represent Him imperfectly.
Reply to Objection 2. In the significance of names, that from which the name is derived is different sometimes from what it is intended to signify, as for instance, this name "stone" [lapis] is imposed from the fact that it hurts the foot [loedit pedem], but it is not imposed to signify that which hurts the foot, but rather to signify a certain kind of body; otherwise everything that hurts the foot would be a stone [This refers to the Latin etymology of the word "lapis" which has no place in English]. So we must say that these kinds of divine names are imposed from the divine processions; for as according to the diverse processions of their perfections, creatures are the representations of God, although in an imperfect manner; so likewise our intellect knows and names God according to each kind of procession; but nevertheless these names are not imposed to signify the procession themselves, as if when we say "God lives," the sense were, "life proceeds from Him"; but to signify the principle itself of things, in so far as life pre-exists in Him, although it pre-exists in Him in a more eminent way than can be understood or signified.
Reply to Objection 3. We cannot know the essence of God in this life, as He really is in Himself; but we know Him accordingly as He is represented in the perfections of creatures; and thus the names imposed by us signify Him in that manner only.
Source: Summa Theologica (New Advent)