To the objection of the Arians, that two Gods are introduced by a unity of substance, the answer is that a plurality of Gods is more likely to be inferred from diversity of substance. Further, their charge recoils upon themselves. Manifold diversity is the reason why two men cannot be said to be one man, though all men are called individually man, where a unity of nature is referred to. There is one nature alone in them, but there is wholly a unity in the Divine Persons. Therefore the Son is not to be severed from the Father, especially as they dare not deny that worship is due to Him.
39. But the Arians maintain the following: If you say that, as the Father is the only true God, so also is the Son, and confess that the Father and the Son are both of one substance, you introduce not one God, but two. For they who are of one substance seem not to be one God but two Gods. Just as two men or two sheep or more are spoken of, but a man and a sheep are not spoken of as two men or two sheep, but as one man and one sheep.
40. This is what the Arians say; and by this cunning argument they attempt to catch the more simple-minded. However if we read the divine Scriptures we shall find that plurality occurs rather among those things which are of a diverse and different substance, that is, ἑτερούσια. We have this set forth in the books of Solomon, in that passage in which he said: “There are three things impossible to understand, yea, a fourth which I know not, the track of an eagle in the air, the way of a serpent upon a rock, the path of a ship in the sea, and the way of a man in his youth.” An eagle and a ship and a serpent are not of one family and nature, but of a distinguishable and different substance, and yet they are three. On the testimony of Scripture, therefore, they learn that their arguments are against themselves.
41. Therefore, in saying that the substance of the Father and of the Son is diverse and their Godhead distinguishable, they themselves assert there are two Gods. But we, when we confess the Father and the Son, in declaring them still to be of one Godhead, say that there are not two Gods, but one God. And this we establish by the word of the Lord. For where there are several, there is a difference either of nature or of will and work. Lastly, that they may be refuted on their own witness, two men are mentioned: But though they are of one nature by right of birth, yet in time and thought and work and place, they are apart; and so one man cannot be spoken of under the signification and number of two; for there is no unity where there is diversity. But God is said to be one, and the glory and completeness of the Father, the Son, and the Holy Spirit is thus expressed.
42. Such, indeed, is the truth of unity that, when the nature alone of human birth or of human flesh is indicated, one man is the term used for the many, as it is written: “The Lord is my helper, I will not fear what man can do unto me;” that is, not the one person of a man, but the one flesh, the one frailty of human birth. It added also: “It is better to trust in the Lord than to trust in man.” Here, too, it did not denote one particular man, but a universal condition. Then, immediately after it added, speaking of many: “It is better to put confidence in the Lord than to put confidence in princes.” Where man is spoken of, as we have already said, there the common unity of the nature, which exists between all is indicated; but where the princes are mentioned, there is a certain distinction between their different powers.
43. Amongst men, or in men, there exists a unity in some one thing, either in love, or desire, or flesh, or devotion, or faith. But a universal unity, that embraces within itself all things agreeably to the divine glory, is the property of the Father, the Son, and the Holy Spirit alone.
44. Wherefore the Lord also, in pointing out the diversity that exists among men, who have nothing in common that can tend towards the unity of an indivisible substance, says: “In your law it is written that the testimony of two men is true.” But though He had said, “The testimony of two men is true,” when He came to the testimony of Himself and His Father, He said not: “Our testimony is true, for it is the testimony of two Gods;” but: “I am One that bear witness of Myself, and the Father that sent Me bears witness of Me.” Earlier He also says: “If I judge, My judgment is true; for I am not alone, but I and the Father that sent Me.” Thus, both in one place and the other, He indicated both the Father and the Son, but neither implied the plurality, nor severed the unity of their divine Substance.
45. It is plain, then, that whatsoever is of one substance cannot be severed, even though it be not single, but one. By singleness I mean that which the Greeks call μονοτής. Singleness has to do with a person; unity with a nature. That those things which are of a different substance are wont to be called, not one alone, but many, though already proved on the testimony of the prophet, the Apostle himself has stated in so many words, saying: “For though there be that are called gods, whether in heaven or in earth.” Do you see, then, that those who are of different substances, and not of the verity of one nature, are called “gods”? But the Father and the Son, being of one substance, are not two Gods, but “One God, the Father, of Whom are all things, and one Lord Jesus Christ, through Whom are all things.” “One God,” he says, “and one Lord Jesus;” and above: “One God, not two Gods;” and then: “One Lord, not two Lords.”
46. Plurality, therefore, is excluded, but the unity is not destroyed. But as, on the one hand, when we read of the Lord Jesus, we do not dissociate the Father, as I have already said, from the prerogative of ruling, because He has that in common with the Son; so, on the other hand, when we read of the only true God, the Father, we cannot sever the Son from the prerogative of the only true God, for He has that in common with the Father.
47. Let them say what they feel or what they think, when we read: “You shall worship the Lord your God, and Him only shall you serve.” Do they think Christ should not be worshipped, and that He ought not to be served? But if that woman of Canaan who worshipped Him, merited to gain what she asked for, and the Apostle Paul, who confessed himself to be the servant of Christ in the very outset of his letters, merited to be an Apostle “not of men, neither by man, but by Jesus Christ;” let them say what they think should follow. Would they prefer to join with Arius in a league of treachery, and so show, by denying Christ to be the only true God, that they consider He should neither be worshipped nor served? Or would they sooner go in company with Paul, who in serving and worshipping Christ did not disown in word and heart the only true God, Whom he acknowledged with dutiful service?
Source: On the Christian Faith (De fide) (New Advent)