5 God is a sort of substance: for that which is no substance, is nothing at all. To be a substance then is to be something. Whence also in the Catholic Faith against the poisons of certain heretics thus we are built up, so that we say, Father and Son and Holy Spirit are of one substance. What is, of one substance? For example, if gold is the Father, gold is also the Son, gold also the Holy Spirit. Whatever the Father is because He is God, the same is the Son, the same the Holy Spirit.
But when He is the Father, this is not what He is. For Father He is called not in reference to Himself, but in reference to the Son: but in reference to Himself God He is called. Therefore in that He is God, by the same He is a substance. And because of the same substance the Son is, without doubt the Son also is God. But yet in that He is Father, because it is not the name of the substance, but is referred to the Son; we do not say that the Son is Father in the same manner as we say the Son is God. You ask what the Father is; we answer, God. You ask what is the Father and the Son: we answer, God.
If questioned of the Father alone, answer God: if questioned of both, not Gods, but God, answer thou. We do not reply as in the case of men, when you inquire what is father Abraham, we answer a man; the substance of him serves for answer: you inquire what is his son Isaac, we answer, a man; of the same substance are Abraham and Isaac: you inquire what is Abraham and Isaac, we answer not man, but men. Not so in things divine. For so great in this case is the fellowship of substance, that of equality it allows, plurality allows not.
If then it shall have been said to you, when you tell me that the Son is the same as the Father, in fact the Son also is the Father; answer thou, according to the substance I have told you that the Son is the same as the Father, not according to that term which is used in reference to something else. For in reference to Himself He is called God, in reference to the Father is called Son. And again, the Father in reference to Himself is called God, in reference to the Son He is called Father.
The Father as He is called in reference to the Son, is not the Son: the Son as He is called in reference to the Father, is not the Father: what the Father is called in reference to Himself and the Son in reference to Himself, the same is Father and Son, that is, God. What is then, “there is no substance”? After this interpretation of substance, how shall we be able to understand this passage of the Psalm, “Fixed I am in the clay of the deep, and there is no substance”? God made man, He made substance; and O that he had continued in that which God made Him!
If man had continued in that which God made him, in him would not have been fixed He whom God begot. But moreover because through iniquity man fell from the substance wherein he was made (for iniquity itself is no substance; for iniquity is not a nature which God formed, but a perverseness which man made); the Son of God came to the clay of the deep, and was fixed; and that was no substance wherein He was fixed, because in the iniquity of them He was fixed. “All things by Him were made, and without Him there was made nothing.” All natures by Him were made, iniquity by Him was not made, because iniquity was not made. Those substances by Him were made, which praise Him.
The whole creation praising God is commemorated by the three children in the furnace, and from things earthly to things heavenly, or from things heavenly to things earthly reaches the hymn of them praising God. Not that all these things have sense to praise; but because all things being well meditated upon, do beget praise, and the heart by considering creation is fulfilled to overflowing with a hymn to the Creator. All things do praise God, but only the things which God has made.
Do ye observe in that hymn that covetousness praises God? There even the serpent praises God, covetousness praises not. For all creeping things are there named in the praise of God: there are named all creeping things; but there are not there named any vices. For vices out of ourselves and out of our own will we have: and vices are not a substance. In these was fixed the Lord, when He suffered persecution: in the vice of the Jews, not in the substance of men which by Him was made.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)