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FIRST ARTICLE [I, Q. 31, Art. 1]

Whether There Is Trinity in God?

Objection 1: It would seem there is not trinity in God. For every name
in God signifies substance or relation. But this name "Trinity" does
not signify the substance; otherwise it would be predicated of each
one of the persons: nor does it signify relation; for it does not
express a name that refers to another. Therefore the word "Trinity" is
not to be applied to God.

Obj. 2: Further, this word "trinity" is a collective term, since it
signifies multitude. But such a word does not apply to God; as the
unity of a collective name is the least of unities, whereas in God
there exists the greatest possible unity. Therefore this word
"trinity" does not apply to God.

Obj. 3: Further, every trine is threefold. But in God there is not
triplicity; since triplicity is a kind of inequality. Therefore
neither is there trinity in God.

Obj. 4: Further, all that exists in God exists in the unity of the
divine essence; because God is His own essence. Therefore, if Trinity
exists in God, it exists in the unity of the divine essence; and thus
in God there would be three essential unities; which is heresy.

Obj. 5: Further, in all that is said of God, the concrete is
predicated of the abstract; for Deity is God and paternity is the
Father. But the Trinity cannot be called trine; otherwise there would
be nine realities in God; which, of course, is erroneous. Therefore
the word trinity is not to be applied to God.

_On the contrary,_ Athanasius says: "Unity in Trinity; and Trinity in
Unity is to be revered."

_I answer that,_ The name "Trinity" in God signifies the determinate
number of persons. And so the plurality of persons in God requires
that we should use the word trinity; because what is indeterminately
signified by plurality, is signified by trinity in a determinate
manner.

Reply Obj. 1: In its etymological sense, this word "Trinity" seems to
signify the one essence of the three persons, according as trinity
may mean trine-unity. But in the strict meaning of the term it rather
signifies the number of persons of one essence; and on this account
we cannot say that the Father is the Trinity, as He is not three
persons. Yet it does not mean the relations themselves of the
Persons, but rather the number of persons related to each other; and
hence it is that the word in itself does not express regard to
another.

Reply Obj. 2: Two things a