His acknowledgment that the Divine Being is 'single' is only verbal
“Each Being has, in fact and in conception, a nature unmixed, single, and absolutely one as estimated by its dignity; and as the works are bounded by the energies of each operator, and the energies by the works, it is inevitable that the energies which follow each Being are greater in the one case than the other, some being of the first, others of the second rank.” The intention that runs through all this, however verbosely expressed, is one and the same; namely, to establish that there is no connection between the Father and the Son, or between the Son and the Holy Ghost, but that these Beings are sundered from each other, and possess natures foreign and unfamiliar to each other, and differ not only in that, but also in magnitude and in subordination of their dignities, so that we must think of one as greater than the other, and presenting every other sort of difference.
It may seem to many useless to linger over what is so obvious, and to attempt a discussion of that which to them is on the face of it false and abominable and groundless: nevertheless, to avoid even the appearance of having to let these statements pass for want of counter-arguments, we will meet them with all our might. He says, “each being among them is unmixed, single, and absolutely one, as estimated by its dignity, both in fact and in conception.” Then premising this very doubtful statement as an axiom and valuing his own 'ipse dixit' as a sufficient substitute for any proof, he thinks he has made a point. “There are three Beings:” for he implies this when he says, 'each being among them:' he would not have used these words, if he meant only one. Now if he speaks thus of the mutual difference between the Beings in order to avoid complicity with the heresy of Sabellius, who applied three titles to one subject, we would acquiesce in his statement: nor would any of the Faithful contradict his view, except so far as he seems to be at fault in his names, and his mere form of expression in speaking of 'beings' instead of 'persons:' for things that are identical on the score of being will not all agree equally in definition on the score of personality. For instance, Peter, James, and John are the same viewed as beings, each was a man: but in the characteristics of their respective personalities, they were not alike. If, then, he were only proving that it is not right to confound the Persons, and to fit all the three names on to one Subject, his 'saying' would be, to use the Apostle's words, 'faithful, and worthy of all acceptation.' But this is not his object: he speaks so, not because he divides the Persons only from each other by their recognized characteristics, but because he makes the actual substantial being of each different from that of the others, or rather from itself: and so he speaks of a plurality of beings with distinctive differences which alienate them from each other. I therefore declare that his view is unfounded, and lacks a principle: it starts from data that are not granted, and then it constructs by mere logic a blasphemy upon them. It attempts no demonstration that could attract towards such a conception of the doctrine: it merely contains the statement of an unproved impiety, as if it were telling us a dream. While the Church teaches that we must not divide our faith among a plurality of beings, but must recognize no difference of being in three Subjects or Persons, whereas our opponents posit a variety and unlikeness among them as Beings, this writer confidently assumes as already proved what never has been, and never can be, proved by argument: maybe he has not even yet found hearers for his talk: or he might have been informed by one of them who was listening intelligently that every statement which is made at random, and without proof, is 'an old woman's tale,' and powerless to prove the question, in itself, unaided by any plea whatever fetched from the Scriptures, or from human reasonings. So much for this.
But let us still scrutinize his words. He declares each of these Beings, whom he has shadowed forth in his exposition, to be single and absolutely one. We believe that the most boorish and simple-minded would not deny that the Divine Nature, blessed and transcendent as it is, was 'single.' That which is viewless, formless, and sizeless, cannot be conceived of as multiform and composite. But it will be clear, upon the very slightest reflection, that this view of the supreme Being as 'simple,' however finely they may talk of it, is quite inconsistent with the system which they have elaborated. For who does not know that, to be exact, simplicity in the case of the Holy Trinity admits of no degrees. In this case there is no mixture or conflux of qualities to think of; we comprehend a potency without parts and composition; how then, and on what grounds, could any one perceive there any differences of less and more. For he who marks differences there must perforce think of an incidence of certain qualities in the subject. He must in fact have perceived differences in largeness and smallness therein, to have introduced this conception of quantity into the question: or he must posit abundance or diminution in the matter of goodness, strength, wisdom, or of anything else that can with reverence be associated with God: and neither way will he escape the idea of composition. Nothing which possesses wisdom or power or any other good, not as an external gift, but rooted in its nature, can suffer diminution in it; so that if any one says that he detects Beings greater and smaller in the Divine Nature, he is unconsciously establishing a composite and heterogeneous Deity, and thinking of the Subject as one thing, and the quality, to share in which constitutes as good that which was not so before, as another. If he had been thinking of a Being really single and absolutely one, identical with goodness rather than possessing it, he would not be able to count a greater and a less in it at all. It was said, moreover, above that good can be diminished by the presence of evil alone, and that where the nature is incapable of deteriorating, there is no limit conceived of to the goodness: the unlimited, in fact, is not such owing to any relation whatever, but, considered in itself, escapes limitation. It is, indeed, difficult to see how a reflecting mind can conceive one infinite to be greater or less than another infinite. So that if he acknowledges the supreme Being to be 'single' and homogenous, let him grant that it is bound up with this universal attribute of simplicity and infinitude. If, on the other hand, he divides and estranges the 'Beings' from each other, conceiving that of the Only-begotten as another than the Father's, and that of the Spirit as another than the Only-begotten, with a 'more' and 'less' in each case, let him be exposed now as granting simplicity in appearance only to the Deity, but in reality proving the composite in Him.
Source: Against Eunomius (New Advent)