Objection 1. It would seem that the numeral terms denote something real in God. For the divine unity is the divine essence. But every number is unity repeated. Therefore every numeral term in God signifies the essence; and therefore it denotes something real in God.
Objection 2. Further, whatever is said of God and of creatures, belongs to God in a more eminent manner than to creatures. But the numeral terms denote something real in creatures; therefore much more so in God.
Objection 3. Further, if the numeral terms do not denote anything real in God, and are introduced simply in a negative and removing sense, as plurality is employed to remove unity, and unity to remove plurality; it follows that a vicious circle results, confusing the mind and obscuring the truth; and this ought not to be. Therefore it must be said that the numeral terms denote something real in God.
On the contrary, Hilary says (De Trin. iv): "If we admit companionship"--that is, plurality--"we exclude the idea of oneness and of solitude;" and Ambrose says (De Fide i): "When we say one God, unity excludes plurality of gods, and does not imply quantity in God." Hence we see that these terms are applied to God in order to remove something; and not to denote anything positive.
I answer that, The Master (Sent. i, D, 24) considers that the numeral terms do not denote anything positive in God, but have only a negative meaning. Others, however, assert the contrary.
In order to resolve this point, we may observe that all plurality is a consequence of division. Now division is twofold; one is material, and is division of the continuous; from this results number, which is a species of quantity. Number in this sense is found only in material things which have quantity. The other kind of division is called formal, and is effected by opposite or diverse forms; and this kind of division results in a multitude, which does not belong to a genus, but is transcendental in the sense in which being is divided by one and by many. This kind of multitude is found only in immaterial things.
Some, considering only that multitude which is a species of discrete quantity, and seeing that such kind of quantity has no place in God, asserted that the numeral terms do not denote anything real in God, but remove something from Him. Others, considering the same kind of multitude, said that as knowledge exists in God according to the strict sense of the word, but not in the sense of its genus (as in God there is no such thing as a quality), so number exists in God in the proper sense of number, but not in the sense of its genus, which is quantity.
But we say that numeral terms predicated of God are not derived from number, a species of quantity, for in that sense they could bear only a metaphorical sense in God, like other corporeal properties, such as length, breadth, and the like; but that they are taken from multitude in a transcendent sense. Now multitude so understood has relation to the many of which it is predicated, as "one" convertible with "being" is related to being; which kind of oneness does not add anything to being, except a negation of division, as we saw when treating of the divine unity (11, 1); for "one" signifies undivided being. So, of whatever we say "one," we imply its undivided reality: thus, for instance, "one" applied to man signifies the undivided nature or substance of a man. In the same way, when we speak of many things, multitude in this latter sense points to those things as being each undivided in itself.
But number, if taken as a species of quantity, denotes an accident added to being; as also does "one" which is the principle of that number. Therefore the numeral terms in God signify the things of which they are said, and beyond this they add negation only, as stated (Sent. i, D, 24); in which respect the Master was right (Sent. i, D, 24). So when we say, the essence is one, the term "one" signifies the essence undivided; and when we say the person is one, it signifies the person undivided; and when we say the persons are many, we signify those persons, and their individual undividedness; for it is of the very nature of multitude that it should be composed of units.
Reply to Objection 1. One, as it is a transcendental, is wider and more general than substance and relation. And so likewise is multitude; hence in God it may mean both substance and relation, according to the context. Still, the very signification of such names adds a negation of division, beyond substance and relation; as was explained above.
Reply to Objection 2. Multitude, which denotes something real in creatures, is a species of quantity, and cannot be used when speaking of God: unlike transcendental multitude, which adds only indivision to those of which it is predicated. Such a kind of multitude is applicable to God.
Reply to Objection 3. "One" does not exclude multitude, but division, which logically precedes one or multitude. Multitude does not remove unity, but division from each of the individuals which compose the multitude. This was explained when we treated of the divine unity (11, 2).
It must be observed, nevertheless, that the opposite arguments do not sufficiently prove the point advanced. Although the idea of solitude is excluded by plurality, and the plurality of gods by unity, it does not follow that these terms express this signification alone. For blackness is excluded by whiteness; nevertheless, the term whiteness does not signify the mere exclusion of blackness.
Source: Summa Theologica (New Advent)