I answer that, It follows from what precedes that there are several persons in God. For it was shown above (Q. 29, A. 4) that this word "person" signifies in God a relation as subsisting in the divine nature. It was also established (Q. 28, A. 1) that there are several real relations in God; and hence it follows that there are also several realities subsistent in the divine nature; which means that there are several persons in God.
Reply Obj. 1: The definition of "person" includes "substance," not as meaning the essence, but the suppositum which is made clear by the addition of the term "individual." To signify the substance thus understood, the Greeks use the name "hypostasis." So, as we say, "Three persons," they say "Three hypostases." We are not, however, accustomed to say Three substances, lest we be understood to mean three essences or natures, by reason of the equivocal signification of the term.
Reply Obj. 2: The absolute properties in God, such as goodness and wisdom, are not mutually opposed; and hence, neither are they really distinguished from each other. Therefore, although they subsist, nevertheless they are not several subsistent realities—that is, several persons. But the absolute properties in creatures do not subsist, although they are really distinguished from each other, as whiteness and sweetness; on the other hand, the relative properties in God subsist, and are really distinguished from each other (Q. 28, A. 3). Hence the plurality of persons in God.
Reply Obj. 3: The supreme unity and simplicity of God exclude every kind of plurality of absolute things, but not plurality of relations. Because relations are predicated relatively, and thus the relations do not import composition in that of which they are predicated, as Boethius teaches in the same book.
Reply Obj. 4: Number is twofold, simple or absolute, as two and three and four; and number as existing in things numbered, as two men and two horses. So, if number in God is taken absolutely or abstractedly, there is nothing to prevent whole and part from being in Him, and thus number in Him is only in our way of understanding; forasmuch as number regarded apart from things numbered exists only in the intellect. But if number be taken as it is in the things numbered, in that sense as existing in creatures, one is part of two, and two of three, as one man is part of two men, and two of three; but this does not apply to God, because the Father is of the same magnitude as the whole Trinity, as we shall show further on (Q. 42, AA. 1, 4). _______________________
SECOND ARTICLE [I, Q. 30, Art. 2]
Whether There Are More Than Three Persons in God?
Objection 1: It would seem that there are more than three persons in God. For the plurality of persons in God arises from the plurality of the relative properties as stated above (A. 1). But there are four relations in God as stated above (Q. 28, A. 4), paternity, filiation, common spiration, and procession. Therefore there are four persons in God.
Obj. 2: The nature of God does not differ from His will more than from His intellect. But in God, one person proceeds from the will, as love; and another proceeds from His nature, as Son. Therefore another proceeds from His intellect, as Word, besides the one Who proceeds from His nature, as Son; thus again it follows that there are not only three persons in God.
Obj. 3: Further, the more perfect a creature is, the more interior operations it has; as a man has understanding and will beyond other animals. But God infinitely excels every creature. Therefore in God not only is there a person proceeding from the will, and another from the intellect, but also in an infinite number of ways. Therefore there are an infinite number of persons in God.