That the Unity of the Essence of the Father and the Son is to Be Gathered from the Words, We are One. The Son is Equal to the Father Both in Wisdom and in All Other Things
4 And I know not whether the words, “They are one,” are ever found in Scripture as spoken of things of which the nature is different. But if there are more things than one of the same nature, and they differ in sentiment, they are not one, and that so far as they differ in sentiment. For if the disciples were already one by the fact of being men, He would not say, “That they may be one, as we are one,” when commending them to the Father. But because Paul and Apollos were both alike men, and also of like sentiments, “He that plants,” he says, “and he that waters are one.” When, therefore, anything is so called one, that it is not added in what it is one, and yet more things than one are called one, then the same essence and nature is signified, not differing nor disagreeing. But when it is added in what it is one, it may be meant that something is made one out of things more than one, though they are different in nature. As soul and body are assuredly not one; for, what are so different? Unless there be added, or understood in what they are one, that is, one man, or one animal [person]. Thence the apostle says, “He who is joined to a harlot, is one body;” he does not say, they are one or he is one; but he has added “body,” as though it were one body composed by being joined together of two different bodies, masculine and feminine. And, “He that is joined unto the Lord,” he says, is one spirit: he did not say, he that is joined unto the Lord is one, or they are one; but he added, “spirit.” For the spirit of man and the Spirit of God are different in nature; but by being joined they become one spirit of two different spirits, so that the Spirit of God is blessed and perfect without the human spirit, but the spirit of man cannot be blessed without God. Nor is it without cause, I think, that when the Lord said so much in the Gospel according to John, and so often, of unity itself, whether of His own with the Father, or of ours interchangeably with ourselves; He has nowhere said, that we are also one with Himself, but, “that they maybe one as we also are one.” Therefore the Father and the Son are one, undoubtedly according to unity of substance; and there is one God, and one great, and one wise, as we have argued.
5. Whence then is the Father greater? For if greater, He is greater by greatness; but whereas the Son is His greatness, neither assuredly is the Son greater than He who begot Him, nor is the Father greater than that greatness, whereby He is great; therefore they are equal. For whence is He equal, if not in that which He is, to whom it is not one thing to be, and another to be great? Or if the Father is greater in eternity, the Son is not equal in anything whatsoever. For whence equal? If you say in greatness, that greatness is not equal which is less eternal, and so of all things else. Or is He perhaps equal in power, but not equal in wisdom? But how is that power which is less wise, equal? Or is He equal in wisdom, but not equal in power? But how is that wisdom equal which is less powerful? It remains, therefore, that if He is not equal in anything, He is not equal in all. But Scripture proclaims, that “He thought it not robbery to be equal with God.” Therefore any adversary of the truth whatever, provided he feels bound by authority, must needs confess that the Son is equal with God in each one thing whatsoever. Let him choose that which he will; from it he will be shown, that He is equal in all things which are said of His substance.
Chapter 4.— The Same Argument Continued.
6. For in like manner the virtues which are in the human mind, although each has its own several and different meaning, yet are in no way mutually separable; so that, for instance, whosoever were equal in courage, are equal also in prudence, and temperance, and justice. For if you say that such and such men are equal in courage, but that one of them is greater in prudence, it follows that the courage of the other is less prudent, and so neither are they equal in courage, since the courage of the former is more prudent. And so you will find it to be the case with the other virtues, if you consider them one by one. For the question is not of the strength of the body, but of the courage of the mind. How much more therefore is this the case in that unchangeable and eternal substance, which is incomparably more simple than the human mind is? Since, in the human mind, to be is not the same as to be strong, or prudent, or just, or temperate; for a mind can exist, and yet have none of these virtues. But in God to be is the same as to be strong, or to be just, or to be wise, or whatever is said of that simple multiplicity, or multifold simplicity, whereby to signify His substance. Wherefore, whether we say God of God in such way that this name belongs to each, yet not so that both together are two Gods, but one God; for they are in such way united with each other, as according to the apostle's testimony may take place even in diverse and differing substances; for both the Lord alone is a Spirit, and the spirit of a man alone is assuredly a spirit; yet, if it cleave to the Lord, “it is one spirit:” how much more there, where there is an absolutely inseparable and eternal union, so that He may not seem absurdly to be called as it were the Son of both, when He is called the Son of God, if that which is called God is only said of both together. Or perhaps it is, that whatever is said of God so as to indicate His substance, is not said except of both together, nay of the Trinity itself together? Whether therefore it be this or that (which needs a closer inquiry), it is enough for the present to see from what has been said, that the Son is in no respect equal with the Father, if He is found to be unequal in anything which has to do with signifying His substance, as we have already shown. But the apostle has said that He is equal. Therefore the Son is equal with the Father in all things, and is of one and the same substance.
Source: On the Holy Trinity (New Advent)