we cannot demonstrate that God exists.

Obj. 3: Further, if the existence of God were demonstrated, this
could only be from His effects. But His effects are not proportionate
to Him, since He is infinite and His effects are finite; and between
the finite and infinite there is no proportion. Therefore, since a
cause cannot be demonstrated by an effect not proportionate to it, it
seems that the existence of God cannot be demonstrated.

_On the contrary,_ The Apostle says: "The invisible things of Him are
clearly seen, being understood by the things that are made" (Rom.
1:20). But this would not be unless the existence of God could be
demonstrated through the things that are made; for the first thing we
must know of anything is whether it exists.

_I answer that,_ Demonstration can be made in two ways: One is through
the cause, and is called _a priori,_ and this is to argue from what is
prior absolutely. The other is through the effect, and is called a
demonstration _a posteriori_; this is to argue from what is prior
relatively only to us. When an effect is better known to us than its
cause, from the effect we proceed to the knowledge of the cause. And
from every effect the existence of its proper cause can be
demonstrated, so long as its effects are better known to us; because
since every effect depends upon its cause, if the effect exists, the
cause must pre-exist. Hence the existence of God, in so far as it is
not self-evident to us, can be demonstrated from those of His effects
which are known to us.

Reply Obj. 1: The existence of God and other like truths about
God, which can be known by natural reason, are not articles of faith,