Summa Theologica

Christian doctrine ascribes certain relationships among the persons of the Trinity. Aquinas asserts that these relations are real, not merely logical or metaphorical constructs. Further, in God, relation is not merely accidental but substantive. In other words, relation is identical to God’s nature and essence. Even so, the relations between Father, Son, and Holy Spirit are distinct. There are in fact four relations in the Trinity: paternity, filiation, spiration, and procession.

Aquinas begins his discussion of the persons of the Trinity by discussing personhood in general. What is a person? Aquinas starts with the definition offered by Boethius: “an individual substance of a rational nature” (162).

“Person” is that which is most excellent in all nature. This being so, the name “person” is rightfully applied to God, but in a more excellent way than it is applied to creatures. Namely, God’s personhood consists in his being supreme intelligent self-subsisting being (165). Unlike creatures, God’s personhood is not given individuality by matter. As applied to the Godhead, the word person denotes relation, referring to the relationship between the three persons of the Trinity. 

Aquinas argues through logic that there are and can be no more than three persons in God. The numerical terms we apply to God (e.g., He is one and He has three persons) are used mostly in a limiting, negative sense: Thus, “one” excludes the idea of multiple gods, while “three” excludes the idea that God is alone in his Godhead. These numbers apply to God in a transcendental sense, referring to his undividedness, but not in a sense that relates to quantity, as we apply it to material things. 

Aquinas continues to explore the Trinity in its various shades of meaning. We say that God is a “Trinity” rather than a “triplicity” because the former term describes the equal-and-unified nature of the three persons in God.

The three persons of the Trinity are distinct from each other, yet each is equally God. 

The truth of the Trinity cannot be known through natural reason. Religious mysteries cannot be proven by rational arguments, but arguments can help confirm their truth.

In addition to the realities of the three persons of the Trinity, their properties and relationships may be expressed by various “notions”:

1.  innascibility (meaning the Father was not born or created; equivalent to unbegotten)

2.  paternity

3.  sonship

4.  common spiration

5.  procession

Aquinas allows that different opinions concerning the notions may be held without constituting heresy. 

God’s fatherhood applies to the Son first, and then to creatures. This is because God is Father to the Son from eternity, while he is Father to creatures in time. Our human sonship to God is applied insofar as it participates in the sonship of the Son of God.

The Father and Son may also be referred to as “principles,” meaning “that from which another proceeds” (181). The Father is “the principle not from a principle” and the Son is “the principle from a principle” (184) as well as the principle of the Holy Spirit. 

The Son of God is called the “Word of God” in a personal, and not merely a metaphorical, sense. Only the Son—and not the Father or the Holy Spirit—is called Word, because he emanates from the intellect of the Father, similar to how human beings speak a word that we have conceived in our mind. The Word of God, however, as a phrase also relates to creatures, because God with his intellect understands everything that he has made. 

“Image” is another personal name applied to the Son, meaning that he bears a likeness to and shares his essential nature with the Father. However, human beings are also said to be the image of God, albeit in a more imperfect sense. Aquinas asserts that it is better to say that man is “to the image of God,” meaning that he has a tendency toward the perfection implied in the term. 

Having discussed the Father and Son, Aquinas now considers the third person of the Holy Trinity, the Holy Spirit (Holy Ghost). The Holy Spirit is the most difficult of the Holy Persons to discuss, especially since there is little terminology associated with him in scripture. Aquinas’ discussion centers on the meaning of the term “Holy Spirit” and on demonstrating the precise relationship of the Spirit toward the Father and the Son. The word “Spirit” does not have relational content as do the terms “Father” and “Son,” but is still a fitting term for the Third Holy Person since it suggests impulse and motion, as the motion that impels us to love God. 

This is the first of two questions dealing with traditional names applied to the Holy Spirit. Aquinas argues that “Love” is the proper name for the Holy Spirit, just as “Word” is the proper name of the Son.

There are two processions that occur in the persons of the Trinity: by way of the intellect (the procession of the Word) and by way of the will (the procession of the Spirit). We know from our human experience that the will is the faculty involved in loving: We love something that we have first known. Aquinas’ argument turns on the idea that love is both what exists in the will and what proceeds from the will as action. Thus, the Holy Spirit, which proceeds from God, can be called the Love of God.

In addition, Aquinas asserts that the Father loves the Son through the Holy Spirit. The Holy Spirit is thus the bond between the Father and Son. 

Having explained why “Love” is a proper name for the Holy Spirit, Aquinas now explains why “Gift” is likewise. Recalling Aristotle, Aquinas defines gift as a free, unreturnable giving motivated by love. In this act of giving, love itself is the first gift, through which all other gifts are given. Thus, the Holy Spirit, as the procession of Love from God, is the first gift to us. By this primal gift of the Spirit, innumerable particular gifts are given to us, the members of Christ’s body.

Aquinas wishes to avoid two heretical extremes: on the one hand, the view that the persons of the Trinity are separate Gods; and on the other hand, the tendency to collapse the three persons into mere “modes” of God’s being. Thus, Aquinas declares that God has one essence and three persons (203). 

Aquinas argues that it is in God’s very essence to be relational. God’s essence is the same as his person, and thus in God relation is the same as person. 

By “notions” Aquinas means the distinctive characteristics of the persons of the Holy Trinity insofar as they are known by us. (First discussed in Question 32, these include qualities such as innascibility, paternity, and sonship.) Such qualities are not voluntary, meaning they do not come about by God’s will in the sense in which he wills to make creatures. They do not belong to the class of contingent things. Rather, they belong to the class of nature—those things that can be only one way. This is because God is necessary being. Thus, we must say that God begot the Son by nature, not by will.

By declaring this, Aquinas wishes to avoid the Arian heresy implying that the Son is a creature. Unlike created things, the Father begets the Son from his substance rather than creating the Son from nothing. 

The three Persons of the Trinity are equal, inasmuch as no one of them precedes in eternity or excels in greatness or surpasses in power (225). They are co-eternal, and there was never a time when one of them did not exist. The Father is in the Son and the Son in the Father. The Son derives his power from the Father, with whom he shares the same nature. 

Having discussed the Trinity as it exists in itself, Aquinas now beings to transition to his Treatise on Creation by considering how the Trinity interacts with the world.

Divine persons—the Son and the Holy Spirit—are rightly said to be sent on a mission in the world. Such a mission involves two elements: the relation of the one sent to the sender, and its relation to the place to which it is sent. Once sent, the divine persons exist in a new way in us—Aquinas stresses in a new way, because God is after all present everywhere to begin with. A change takes place in us, rather than in God. This mission comes about only through God’s sanctifying grace, because only this can allow us to possess a divine person. The mission of the divine persons was sometimes made manifest in visible form, as at Christ’s baptism and at Pentecost.