7 As for this threefold or rather fourfold fiction, what shall I say of the secret light of the Father, but that you can think of no light except what you have seen? From your knowledge of visible light, with which beasts and insects as well as men are familiar, you form some vague idea in your mind, and call it the light in which God the Father dwells with His subjects. How can you distinguish between the light by which we see, and that by which we understand, when, according to your ideas, to understand truth is nothing else than to form the conception of material forms, either finite or in some cases infinite; and you actually believe in these wild fancies?
It is manifest that the act of my mind in thinking of your region of light which has no existence, is entirely different from my conception of Alexandria, which exists, though I have not seen it. And, again, the act of forming a conception of Alexandria, which I have never seen, is very different from thinking of Carthage, which I know. But this difference is insignificant as compared with that between my thinking of material things which I know from seeing them, and my understanding justice, chastity, faith, truth, love, goodness, and things of this nature.
Can you describe this intellectual light, which gives us a clear perception of the distinction between itself and other things, as well as of the distinction between those things themselves? And yet even this is not the sense in which it can be said that God is light, for this light is created, whereas God is the Creator; the light is made, and He is the Maker; the light is changeable. For the intellect changes from dislike to desire, from ignorance to knowledge, from forgetfulness to recollection; whereas God remains the same in will, in truth, and in eternity.
From God we derive the beginning of existence, the principle of knowledge, the law of affection. From God all animals, rational and irrational, derive the nature of their life, the capacity of sensation, the faculty of emotion. From God all bodies derive their subsistence in extension, their beauty in number, and their order in weight. This light is one divine being, in an inseparable triune existence; and yet, without supposing the assumption of any bodily form, you assign to separate places parts of the immaterial, spiritual, and unchangeable substance.
And instead of three places for the Trinity, you have four: one, the light inaccessible, which you know nothing about, for the Father; two, the sun and moon, for the Son; and again one, the circle of the atmosphere, for the Holy Spirit. Of the inaccessible light of the Father I shall say nothing further at present, for orthodox believers do not separate the Son and the Spirit from the Father in relation to this light.
Source: Reply to Faustus the Manichaean (New Advent)