From On Christian Doctrine:

6. Have I spoken of God, or uttered His praise, in any worthy way? Nay, I feel that I have done nothing more than desire to speak; and if I have said anything, it is not what I desired to say. How do I know this, except from the fact that God is unspeakable? But what I have said, if it had been unspeakable, could not have been spoken. And so God is not even to be called unspeakable, because to say even this is to speak of Him. Thus there arises a curious contradiction of words, because if the unspeakable is what cannot be spoken of, it is not unspeakable if it can be called unspeakable. And this opposition of words is rather to be avoided by silence than to be explained away by speech. And yet God, although nothing worthy of His greatness can be said of Him, has condescended to accept the worship of men’s mouths, and has desired us through the medium of our own words to rejoice in His praise. For on this principle it is that He is called Deus (God). For the sound of those two syllables in itself conveys no true knowledge of His nature; but yet all who know the Latin tongue are led, when that sound reaches their ears, to think of a nature supreme in excellence and eternal in existence.