Objection 1. It seems that no name can be given to God. For Dionysius says (Div. Nom. i) that, "Of Him there is neither name, nor can one be found of Him;" and it is written: "What is His name, and what is the name of His Son, if thou knowest?" (Proverbs 30:4).
Objection 2. Further, every name is either abstract or concrete. But concrete names do not belong to God, since He is simple, nor do abstract names belong to Him, forasmuch as they do not signify any perfect subsisting thing. Therefore no name can be said of God.
Objection 3. Further, nouns are taken to signify substance with quality; verbs and participles signify substance with time; pronouns the same with demonstration or relation. But none of these can be applied to God, for He has no quality, nor accident, nor time; moreover, He cannot be felt, so as to be pointed out; nor can He be described by relation, inasmuch as relations serve to recall a thing mentioned before by nouns, participles, or demonstrative pronouns. Therefore God cannot in any way be named by us.
On the contrary, It is written (Exodus 15:3): "The Lord is a man of war, Almighty is His name."
I answer that, Since according to the Philosopher (Peri Herm. i), words are signs of ideas, and ideas the similitude of things, it is evident that words relate to the meaning of things signified through the medium of the intellectual conception. It follows therefore that we can give a name to anything in as far as we can understand it. Now it was shown above (12, 11, 12) that in this life we cannot see the essence of God; but we know God from creatures as their principle, and also by way of excellence and remotion. In this way therefore He can be named by us from creatures, yet not so that the name which signifies Him expresses the divine essence in itself. Thus the name "man" expresses the essence of man in himself, since it signifies the definition of man by manifesting his essence; for the idea expressed by the name is the definition.
Reply to Objection 1. The reason why God has no name, or is said to be above being named, is because His essence is above all that we understand about God, and signify in word.
Reply to Objection 2. Because we know and name God from creatures, the names we attribute to God signify what belongs to material creatures, of which the knowledge is natural to us. And because in creatures of this kind what is perfect and subsistent is compound; whereas their form is not a complete subsisting thing, but rather is that whereby a thing is; hence it follows that all names used by us to signify a complete subsisting thing must have a concrete meaning as applicable to compound things; whereas names given to signify simple forms, signify a thing not as subsisting, but as that whereby a thing is; as, for instance, whiteness signifies that whereby a thing is white. And as God is simple, and subsisting, we attribute to Him abstract names to signify His simplicity, and concrete names to signify His substance and perfection, although both these kinds of names fail to express His mode of being, forasmuch as our intellect does not know Him in this life as He is.
Reply to Objection 3. To signify substance with quality is to signify the "suppositum" with a nature or determined form in which it subsists. Hence, as some things are said of God in a concrete sense, to signify His subsistence and perfection, so likewise nouns are applied to God signifying substance with quality. Further, verbs and participles which signify time, are applied to Him because His eternity includes all time. For as we can apprehend and signify simple subsistences only by way of compound things, so we can understand and express simple eternity only by way of temporal things, because our intellect has a natural affinity to compound and temporal things. But demonstrative pronouns are applied to God as describing what is understood, not what is sensed. For we can only describe Him as far as we understand Him. Thus, according as nouns, participles and demonstrative pronouns are applicable to God, so far can He be signified by relative pronouns.
Source: Summa Theologica (New Advent)