From De Trinitate, Book I, where he gives the account of how the combination of his Neoplatonic reflections and the reading of Scripture led him to Christianity:
Therefore, although my soul drew joy from the apprehension of this august and unfathomable Mind, because it could worship as its own Father and Creator so limitless an Infinity, yet with a still more eager desire it sought to know the true aspect of its infinite and eternal Lord, that it might be able to believe that that immeasurable Deity was apparelled in splendour befitting the beauty of His wisdom. Then, while the devout soul was baffled and astray through its own feebleness, it caught from the prophet's voice this scale of comparison for God, admirably expressed, By the greatness of His works and the beauty of the things that He has made the Creator of worlds is rightly discerned. The Creator of great things is supreme in greatness, of beautiful things in beauty. Since the work transcends our thoughts, all thought must be transcended by the Maker. Thus heaven and air and earth and seas are fair: fair also the whole universe, as the Greeks agree, who from its beautiful ordering call it κόσμος, that is, order. But if our thought can estimate this beauty of the universe by a natural instinct — an instinct such as we see in certain birds and beasts whose voice, though it fall below the level of our understanding, yet has a sense clear to them though they cannot utter it, and in which, since all speech is the expression of some thought, there lies a meaning patent to themselves — must not the Lord of this universal beauty be recognised as Himself most beautiful amid all the beauty that surrounds Him? For though the splendour of His eternal glory overtax our mind's best powers, it cannot fail to see that He is beautiful. We must in truth confess that God is most beautiful, and that with a beauty which, though it transcend our comprehension, forces itself upon our perception.
Thus my mind, full of these results which by its own reflection and the teaching of Scripture it had attained, rested with assurance, as on some peaceful watchtower, upon that glorious conclusion, recognising that its true nature made it capable of one homage to its Creator, and of none other, whether greater or less; the homage namely of conviction that His is a greatness too vast for our comprehension but not for our faith. For a reasonable faith is akin to reason and accepts its aid, even though that same reason cannot cope with the vastness of eternal Omnipotence.