God's Eternity - Ch.5 from "The Compendium of Theology" by Thomas Aquinas

Chapter 5 on God's eternity is extremely short, but here are a few things that can be taken from it. 

Eternity is the simultaneous possession of all being and is God's eternal present now. This can be represented in terms of a pyramid in which the bottom line is being spread out in time which rises hierarchically into just a simple point at the top. The point is a good analogy for God's existence as it has no extension. Now one might be asking, what is time? Time, according to Aristotle is the measurement of motion/change. And as Aquinas established previously, God has not change, and thus God will have no time either. 

This also brings up the question about God's actions in history. Doesn't God acting in history mean some type of change for him? Well, properly speaking, all creation and God's action in it is part of God's eternal relation to creation. It is not a temporal event or change. This, of course, has major theological implications; namely that God has known, loved, and willed us from his eternal being. It is not, as we often think when we are young, as though God is "before" creation, like he is sitting around and eventually at some point decides he should make something. This is to misunderstand God's eternity. 

Caput 5 - Quod Deus est aeternus

CHAPTER 5 - THE ETERNITY OF GOD

Ex hoc autem apparet ulterius Deum esse aeternum. Omne enim quod incipit esse vel desinit, per motum vel per mutationem hoc patitur. Ostensum est autem quod Deus est omnino immobilis. Est ergo aeternus.

The further conclusion is evident that God is eternal. For everything that begins to be or that ceases to be, is affected in this way through motion or change. But we have just shown that God is absolutely immobile. Consequently He is eternal.