"Compendium of Theology" Ch. 3 - "The Existence of God" by Thomas Aquinas

A Few Thoughts 
The Compendium of Theology is a summarized version of his work that he explains in more detail in other places. And so here Thomas starts with a summary of the essence of the metaphysical ways to God. At their core, they are based on the universal principle of causality. This principle requires that all reasoning be led to its limit case, i.e. a cause which is perfectly existing of itself and can act as a first cause to all instrumental or secondary causes/beings. There is no other conclusion to which the principle of causality can lead without denying that principle. And the problem is that if one denies the principle of causality, then they have in the very act affirmed it because it too must apply to our thoughts which are based on reality. So if reality is not coherent in terms of causality, then neither are our thoughts or arguments we make against it. The true skeptic must be like Aristotle's vegetable, quiet and unmoving. 

Another key point from the Latin in this chapter is to focus on the word "esse". "Esse" is the infinitive form of the verb "to be" in Latin and is the basis of our attempts in language to understand God. This is because esse is the broadest way of speaking about something. Being is the broadest category of existence or actuality. And so, like in Exodus, when God says that his name is "I AM" it shows that God is perfect being, not some aspect or limitation of being like the pagan gods or any created thing. Zeus is the embodiment of power, Aphrodite of love, Ares of war ... and so on. But these are all limitations and not befitting of the prime mover. Thus, God can only be spoken about as "I AM", as he has no limits to his being. 

The Text 

Caput 3 - CHAPTER 3

Quod Deus sit - THE EXISTENCE OF GOD

Circa essentiae quidem divinae unitatem primo quidem credendum est Deum esse; quod ratione conspicuum est. Videmus enim omnia quae moventur, ab aliis moveri: inferiora quidem per superiora, sicut elementa per corpora caelestia; et in elementis quod fortius est, movet id quod debilius est; et in corporibus etiam caelestibus inferiora a superioribus aguntur. Hoc autem in infinitum procedere impossibile est. Cum enim omne quod movetur ab aliquo, sit quasi instrumentum quoddam primi moventis; si primum movens non sit, quaecumque movent, instrumenta erunt. Oportet autem, si in infinitum procedatur in moventibus et motis, primum movens non esse. Igitur omnia infinita moventia et mota erunt instrumenta. Ridiculum est autem etiam apud indoctos, ponere instrumenta moveri non ab aliquo principali agente: simile enim est hoc ac si aliquis circa constitutionem arcae vel lecti ponat serram vel securim absque carpentario operante. Oportet igitur primum movens esse, quod sit omnibus supremum; et hoc dicimus Deum.

Regarding the unity of the divine essence, we must first believe that God exists. This is a truth clearly known by reason. We observe that all things that move are moved by other things, the lower by the higher. The elements are moved by heavenly bodies; and among the elements themselves, the stronger moves the weaker; and even among the heavenly bodies, the lower are set in motion by the higher. This process cannot be traced back into infinity. For everything that is moved by another is a sort of instrument of the first mover. Therefore, if a first mover is lacking, all things that move will be instruments. But if the series of movers and things moved is infinite, there can be no first mover. In such a case, these infinitely many movers and things moved will all be instruments. But even the unlearned perceive how ridiculous it is to suppose that instruments are moved, unless they are set in motion by some principal agent. This would be like fancying that, when a chest or a bed is being built, the saw or the hatchet performs its functions without the carpenter. Accordingly there must be a first mover that is above all the the rest; and this being we call God.