I ask whether Father and Son and Holy Spirit are predicated of the Godhead substantially, or in some other manner; and I think the way of inquiry must be taken from that point from which the beginning of all things is clearly established, that is, from the very foundations of the catholic faith. If therefore I should ask whether he who is called Father is substance, the answer is that he is substance. And if I should ask whether the Son is substance, the same is said; that the Holy Spirit too is substance, no one would doubt. But when again I gather together Father, Son, Holy Spirit, there occurs to be not several, but one substance;
therefore the one substance of the Three can in no way be separated, nor disjoined, nor is it conjoined into one as if out of parts, but it is simply one. Whatever therefore is predicated of the divine substance, these things must be common to the Three; and this will be the sign of what those things are which are predicated of the substance of the Godhead, namely that whatever is said in this manner of the individuals, when gathered into one, shall be predicated of the Three singly in this manner: if we say, the Father is God, the Son is God, the Holy Spirit is God; the Father, the Son, and the Holy Spirit is one God. If therefore there is one Godhead of them, one substance, the name 'God' is indeed predicated of the Godhead substantially.
Thus the Father is truth, the Son is truth, the Holy Spirit is truth: the Father, the Son, and the Holy Spirit are not three truths, but one truth. If therefore there is one substance in these, there is one truth, and it is necessary that truth be predicated substantially: concerning goodness, concerning immutability, concerning justice, concerning omnipotence, and concerning all the rest, which we predicate singly both of the individuals and of all, it is manifest that they are said substantially. Whence it appears that those things which it is fitting to say of the individuals separately, yet cannot be said of all together, are not predicated substantially, but in another manner: and what that manner may be, I shall inquire afterwards.
For he who is Father does not transmit this term to the Son, nor to the Holy Spirit; whence it comes about that this name is not bestowed as substantial: for if it were substantial, like 'God,' like 'truth,' like 'justice,' like substance itself also, it would be said of the others. Likewise the Son alone receives this name, nor is he joined with the others, as in 'God,' as in 'truth,' as in the rest which I said above. The Holy Spirit too is not the same as the Father and the Son. From these things therefore we understand that Father, Son, and Holy Spirit are not said of the Godhead itself substantially, but in some other manner. For if it were predicated substantially, it would be said both of the individuals and of all singly;
but it is manifest that these are said in relation to something: for the Father is the father of someone; and the Son is the son of someone; and the Spirit is the spirit of someone: whence it comes about that not even the Trinity is predicated of God substantially. For the Father is not the Trinity, for he who is Father is not the Son and the Holy Spirit; nor is the Son the Trinity, nor the Holy Spirit the Trinity according to the same manner; but the Trinity consists indeed in the plurality of persons, while the unity consists in the simplicity of substance. But if the persons are divided, while the substance is undivided, it is necessary that the term which takes its origin from the persons should not pertain to the substance. But the diversity of persons made the Trinity; the Trinity therefore does not pertain to the substance.
Whence it comes about that neither Father, nor Son, nor Holy Spirit, nor the Trinity is predicated of God substantially, but, as has been said, in relation to something. But God, truth, justice, goodness, omnipotence, substance, immutability, virtue, wisdom, and whatever of this kind can be thought of, are said of the Godhead substantially. If these things are rightly and according to faith, I ask that you instruct me; or if perhaps in some respect you are of another mind, look more diligently at what has been said; and join, if you can, faith and reason.