Compendium of Theology Chapter 6: The Necessity of God's Existence - St. Thomas Aquinas

The Infinitive of To Be

This chapter gives a helpful formulation for speaking about God properly. And given that we cannot know what God is directly, such a formulation is invaluable. This formulation is that of God as "ipsum esse", or "being itself". Implied in this is that God is the unlimited action of existing, hence the Latin infinitive of the verb to be, "esse," as opposed to a finite verb wherein a being (ens, entis - the noun) limits the act of existence to this particular type of existence. So God, as a noun, is really the infinitive verb, the unlimited act of existence itself. 

This means that God is necessary existence; God cannot not exist because actual existence is necessarily included in the action of unlimited existence. Or, in other words, when we look at the definitions of all things in this universe, we can see they are "possible" beings. Actual existence is not a necessary part of their definition. We can imagine a world where they do not exist and there's no contradiction. But if all things in the universe are only possible, they would not exist at all on their own. Their existence is possible upon the condition that something else gives them being. Thus there must be some self-existent prime reality which exists, by definition, of necessity. 

CHAPTER 6 - NECESSITY OF GOD’S EXISTENCE

Per hoc autem ostenditur, quod Deum esse sit necessarium. Omne enim quod possibile est esse et non esse, est mutabile. Sed Deus est omnino immutabilis, ut ostensum est. Ergo Deum non est possibile esse et non esse. Omne autem quod est, et non est possibile ipsum non esse, necesse est ipsum esse: quia necesse esse, et non possibile non esse, idem significant. Ergo Deum esse est necesse.

The same line of reasoning clearly shows that God necessarily exists. For everything that has the possibility of being and of not being, is mutable. But God is absolutely immutable, as has been demonstrated. Therefore it is impossible for God to be and not to be. But anything that exists in such a way that it is impossible for it not to exist, is necessarily Being itself, ipsum esse. Necessary existence, and impossibility of nonexistence, mean one and the same thing. Therefore God must necessarily exist.

Item. Omne quod est possibile esse et non esse, indiget aliquo alio quod faciat ipsum esse: quia quantum est in se, se habet ad utrumque. Quod autem facit aliquid esse, est prius eo. Ergo omni quod est possibile esse et non esse, est aliquid prius. Deo autem non est aliquid prius. Ergo non est possibile ipsum esse et non esse, sed necesse est eum esse. Et quia aliqua necessaria sunt quae suae necessitatis causam habent, quam oportet eis esse priorem; Deus, qui est omnium primum, non habet causam suae necessitatis: unde Deum esse per se ipsum est necesse.

Moreover, everything that has a possibility of being and of not being, needs something else to make it be, for, as far as it itself is concerned, it is indifferent with regard to either alternative. But that which causes another thing to be, is prior to that thing. Hence something exists prior to that which has the possibility of being and of not being. However, nothing is prior to God. Therefore it is impossible for Him to be and not to be; of necessity, He must be. And since there are some necessary things that have a cause of their necessity, a cause that must be prior to them, God, who is the first of all, has no cause of His own necessity. Therefore it is necessary for God to be through Himself.