As we have to a certain extent shown by our statement that the word “Godhead” is not significant of nature but of operation, perhaps one might reasonably allege as a cause why, in the case of men, those who share with one another in the same pursuits are enumerated and spoken of in the plural, while on the other hand the Deity is spoken of in the singular as one God and one Godhead, even though the Three Persons are not separated from the significance expressed by the term “Godhead,”— one might allege, I say, the fact that men, even if several are engaged in the same form of action, work separately each by himself at the task he has undertaken, having no participation in his individual action with others who are engaged in the same occupation. For instance, supposing the case of several rhetoricians, their pursuit, being one, has the same name in the numerous cases: but each of those who follow it works by himself, this one pleading on his own account, and that on his own account. Thus, since among men the action of each in the same pursuits is discriminated, they are properly called many, since each of them is separated from the others within his own environment, according to the special character of his operation. But in the case of the Divine nature we do not similarly learn that the Father does anything by Himself in which the Son does not work conjointly, or again that the Son has any special operation apart from the Holy Spirit; but every operation which extends from God to the Creation, and is named according to our variable conceptions of it, has its origin from the Father, and proceeds through the Son, and is perfected in the Holy Spirit. For this reason the name derived from the operation is not divided with regard to the number of those who fulfil it, because the action of each concerning anything is not separate and peculiar, but whatever comes to pass, in reference either to the acts of His providence for us, or to the government and constitution of the universe, comes to pass by the action of the Three, yet what does come to pass is not three things. We may understand the meaning of this from one single instance. From Him, I say, Who is the chief source of gifts, all things which have shared in this grace have obtained their life. When we inquire, then, whence this good gift came to us, we find by the guidance of the Scriptures that it was from the Father, Son, and Holy Spirit. Yet although we set forth Three Persons and three names, we do not consider that we have had bestowed upon us three lives, one from each Person separately; but the same life is wrought in us by the Father, and prepared by the Son, and depends on the will of the Holy Spirit. Since then the Holy Trinity fulfils every operation in a manner similar to that of which I have spoken, not by separate action according to the number of the Persons, but so that there is one motion and disposition of the good will which is communicated from the Father through the Son to the Spirit (for as we do not call those whose operation gives one life three Givers of life, neither do we call those who are contemplated in one goodness three Good beings, nor speak of them in the plural by any of their other attributes); so neither can we call those who exercise this Divine and superintending power and operation towards ourselves and all creation, conjointly and inseparably, by their mutual action, three Gods. For as when we learn concerning the God of the universe, from the words of Scripture, that He judges all the earth, we say that He is the Judge of all things through the Son: and again, when we hear that the Father judges no man, we do not think that the Scripture is at variance with itself—(for He Who judges all the earth does this by His Son to Whom He has committed all judgment; and everything which is done by the Only-begotten has its reference to the Father, so that He Himself is at once the Judge of all things and judges no man, by reason of His having, as we said, committed all judgment to the Son, while all the judgment of the Son is conformable to the will of the Father; and one could not properly say either that They are two judges, or that one of Them is excluded from the authority and power implied in judgment);— so also in the case of the word “Godhead,” Christ is the power of God and the wisdom of God, and that very power of superintendence and beholding which we call Godhead, the Father exercises through the Only-begotten, while the Son perfects every power by the Holy Spirit, judging, as Isaiah says, by the Spirit of judgment and the Spirit of burning, and acting by Him also, according to the saying in the Gospel which was spoken to the Jews. For He says, “If I by the Spirit of God cast out devils”; where He includes every form of doing good in a partial description, by reason of the unity of action: for the name derived from operation cannot be divided among many where the result of their mutual operation is one.
Since, then, the character of the superintending and beholding power is one, in Father, Son, and Holy Spirit, as has been said in our previous argument, issuing from the Father as from a spring, brought into operation by the Son, and perfecting its grace by the power of the Spirit; and since no operation is separated in respect of the Persons, being fulfilled by each individually apart from that which is joined with Him in our contemplation, but all providence, care, and superintendence of all, alike of things in the sensible creation and of those of supramundane nature, and that power which preserves the things which are, and corrects those which are amiss, and instructs those which are ordered aright, is one, and not three, being, indeed, directed by the Holy Trinity, yet not severed by a threefold division according to the number of the Persons contemplated in the Faith, so that each of the acts, contemplated by itself, should be the work of the Father alone, or of the Son peculiarly, or of the Holy Spirit separately, but while, as the Apostle says, the one and the selfsame Spirit divides His good gifts to every man severally, the motion of good proceeding from the Spirit is not without beginning—we find that the power which we conceive as preceding this motion, which is the Only-begotten God, is the maker of all things; without Him no existent thing attains to the beginning of its being: and, again, this same source of good issues from the will of the Father.
If, then, every good thing and every good name, depending on that power and purpose which is without beginning, is brought to perfection in the power of the Spirit through the Only-begotten God, without mark of time or distinction (since there is no delay, existent or conceived, in the motion of the Divine will from the Father, through the Son, to the Spirit): and if Godhead also is one of the good names and concepts, it would not be proper to divide the name into a plurality, since the unity existing in the action prevents plural enumeration. And as the Saviour of all men, specially of them that believe, is spoken of by the Apostle as one, and no one from this phrase argues either that the Son does not save them who believe, or that salvation is given to those who receive it without the intervention of the Spirit; but God who is over all, is the Saviour of all, while the Son works salvation by means of the grace of the Spirit, and yet they are not on this account called in Scripture three Saviours (although salvation is confessed to proceed from the Holy Trinity): so neither are they called three Gods, according to the signification assigned to the term “Godhead,” even though the aforesaid appellation attaches to the Holy Trinity.
Source: On "Not Three Gods" (To Ablabius) (New Advent)