Reply Obj. 2: Perhaps not everyone who hears this word "God"
understands it to signify something than which nothing greater can be
thought, seeing that some have believed God to be a body. Yet, granted
that everyone understands that by this word "God" is signified
something than which nothing greater can be thought, nevertheless, it
does not therefore follow that he understands that what the word
signifies exists actually, but only that it exists mentally. Nor can
it be argued that it actually exists, unless it be admitted that there
actually exists something than which nothing greater can be thought;
and this precisely is not admitted by those who hold that God does not
exist.

Reply Obj. 3: The existence of truth in general is
self-evident but the existence of a Primal Truth is not self-evident
to us.
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SECOND ARTICLE [I, Q. 2, Art. 2]

Whether It Can Be Demonstrated That God Exists?

Objection 1: It seems that the existence of God cannot be
demonstrated. For it is an article of faith that God exists. But what
is of faith cannot be demonstrated, because a demonstration produces
scientific knowledge; whereas faith is of the unseen (Heb. 11:1).
Therefore it cannot be demonstrated that God exists.

Obj. 2: Further, the essence is the middle term of demonstration.
But we cannot know in what God's essence consists, but solely in what
it does not consist; as Damascene says (De Fide Orth. i, 4). Therefore