Compendium of Theology
Chapter 3 – That God exists.
The first thing to be believed about the unity of the Divine Essence is that God exists which is obvious to reason. For we see that everything which is moved is moved by another; for (when) lower things (move) on account of higher ones, as the elements (move) on account of the celestial bodies, the lower are made to act by the higher. But this is impossible to proceed to infinity. For since everything which is moved by another is as though an instrument of the first mover, if there is no first mover, everything which moves will be an instrument. It is necessary, however, that if it were to proceed to infinity in movers and moved things, being no first mover, then all infinite movers and moved things will be instruments. It is ridiculous, even for the uneducated, to posit that instruments are moved without some principle agent. For this is like someone positing a saw or ax for the construction of a chest or a bed without a carpenter using (them). Therefore, it necessary that there be a first mover, which is supreme to everything, and this we call God.
Thomistic Philosophy Page 