The nature of God is paramount in apologetics, so today we will continue to explore what that nature entails. The article below explains God’s eternality:
Eternity (Nontemporality). God is not temporal (Summa Theologica, 1a. 10, 1). He is beyond time. Aquinas offers several arguments in support of this conclusion. The first argument goes:
1. Whatever exists in time can be computed according to its befores and afters.
2. Changeless being, as God is, has no befores or afters; it is always the same.
3. Consequently, God must be timeless.
Time is duration characterized by substantial and accidental changes. A substantial change is a change in what something is. Fire changes what a piece of wood is. An accidental change is a change in what something has. Growing knowledge is an accidental change in a being. Aquinas sees three levels of being in relation to time and eternity:
1. God in eternity is Pure Actuality, without essential or accidental change.
2. Angels and saints who dwell in the spiritual world of heaven live in aeviternity (or aevum).
3. Human beings, comprising soul and body, form and matter, live in time.
Eternity (God) endures without any potency. Aeviternity (angels) endure with completely actualized potency. Their changes are not essential but accidental. Spiritual beings in aeviternity do not change in their essence, though they do undergo accidental changes. Angels increase in knowledge by divine infusion, and they have changeableness with regard to choice, intelligence, affections and places (ibid., 1a.10.6). But with no substantial changes in aeviternity, angels are immutable in their level of grace and charity. What is true of the angels is also true of the elect in heaven.
Time (humanity) endures with progressive actualized potency.
The second argument for God’s eternity similarly follows from immutability. It begins with the premise that whatever is immutable does not change in the state of its being. Whatever is in time goes through a succession of states. So whatever is immutable is not temporal. This argument stresses another aspect of time; whatever is temporal has successive states, one after the other. God does not, so he is not temporal.
Total immutability necessarily implies eternity (ibid., 1a.10.2). For whatever changes substantially is in time and can be computed according to before and after. Whatever does not change cannot be in time, since it has no different states by which before and after can be computed. It never changes. Whatever does not change is not temporal. Not only is God eternal, but he alone is eternal (ibid., 1a.10.3), for he alone is essentially immutable.
Aquinas distinguishes eternity from endless time (ibid., 1a.10.4). First, whatever is essentially whole (eternity) is essentially different from what has parts (time). Eternity is now forever; time includes past, present, and future, now and then. The implication of this is that God’s eternity is not divided; it is all present to him in his eternal now. So it must be essentially different from time in successive moments.
Second, endless time is just more an elongation of time. But eternity differs qualitatively. It differs essentially, not merely accidentally. Eternity is an essential, changeless state of being that transcends moment-by-successive-moment reality. Time measures that reality, or rather the stage on which reality plays out.
Third, an eternal being cannot change, whereas time involves change. By change can the measurements of before and after be made. Whatever can be computed according to before and after is not eternal. Endless time can be computed according to before and after. Hence, endless time is not the same as eternity. The eternal is changeless, but what can be computed by its before and after has changed. It follows, then, that the eternal now cannot live in relation to endless befores and afters.
Obviously, Aquinas saw a crucial difference between the “now” of time and the “now” of eternity (ibid.). The now of time is movable. The now of eternity is not movable in any way. The eternal now is unchanging, but the now of time is ever changing. There is only an analogy between time and eternity; they cannot be the same. God’s now has no past or future; time’s now does.
Some have mistakenly concluded that Aquinas did not believe in God’s duration for eternity, because he rejected temporality in God. Aquinas argued that duration occurs as long as actuality exists. But eternity, aeviternity, and time endure in different ways.
It follows, therefore that the essential difference in the quality of the duration in time, aeviternity, and eternity comes from the condition of the actuality. God is Pure Actuality. Angels have received total actuality from God in their created spiritual forms. Human beings progressively receive actuality in both spiritual form and material body.
Since God endures without potentiality, he cannot endure progressively. He endures in a much higher way—as Pure Actuality.
Geisler, N. L. (1999). In Baker encyclopedia of Christian apologetics (pp. 283–284). Grand Rapids, MI: Baker Books.