Thomas Aquinas

“ Thus it is said that the Lord will reign beyond eternity, inasmuch as He endures beyond every age, i.e. beyond every kind of duration. For age is nothing more than the period of each thing, as is said in the book De Coelo i. Or to reign beyond eternity can be taken to mean that if any other thing were conceived to exist for ever, as the movement of the heavens according to some philosophers, then God would still reign beyond, inasmuch as His reign is simultaneously whole. ”

“ Further, things not dependent on each other do not seem to have one measure of duration; for there appears to be one time for all temporal things; since the first movement, measured by time, is in some way the cause of all movement. But aeviternal things do not depend on each other, for one angel is not the cause of another angel. Therefore there is not only one aeviternity.
On the contrary, Aeviternity is a more simple thing than time, and is nearer to eternity. But time is one only. Therefore much more is aeviternity one only. ”

“ Hence, others assert that the unity of eternity as the principle of all duration is the cause of the unity of time. Thus all durations are one in that view, in the light of their principle, but are many in the light of the diversity of things receiving duration from the influx of the first principle. On the other hand others assign primary matter as the cause why time is one; as it is the first subject of movement, the measure of which is time. ”

“ As eternity is the proper measure of permanent being, so time is the proper measure of movement; and hence, according as any being recedes from permanence of being, and is subject to change, it recedes from eternity, and is subject to time. Therefore the being of things corruptible, because it is changeable, is not measured by eternity, but by time; for time measures not only things actually changed, but also things changeable ”

“ Whatever is possessed, is held firmly and quietly; therefore to designate the immutability and permanence of eternity, we use the word "possession."
Art. 2 - Whether God is eternal? Objection 1: It seems that God is not eternal. For nothing made can be predicated of God; for Boethius says (De Trin. iv) that, "The now that flows away makes time, the now that stands still makes eternity;" and Augustine says (Octog. Tri. Quaest. qu. 28) "that God is the author of eternity." Therefore God is not eternal. ”

“ Yet this is not to be taken as if the negation belonged to their essence, but because our intellect which first apprehends compound things, cannot attain to the knowledge of simple things except by removing the opposite.
Reply to Objection 2: What is truly eternal, is not only being, but also living; and life extends to operation, which is not true of being. Now the protraction of duration seems to belong to operation rather than to being; hence time is the numbering of movement.
Reply to Objection 3: Eternity is called whole, not because it has parts, but because it is wanting in nothing. ”

“ Now to say that an angel was, or is, or will be, is to be taken in a different sense according to the acceptation of our intellect, which apprehends the angelic existence by comparison with different parts of time. ”

“ Further, since aeviternity is a term of duration, where there is one aeviternity, there is also one duration. But not all aeviternal things have one duration, for some begin to exist after others; as appears in the case especially of human souls. ”

“ Aeviternity is sometimes taken for age, that is, a space of a thing's duration; and thus we say many aeviternities when we mean ages.
Reply to Objection 2: Although the heavenly bodies and spiritual things differ in the genus of their nature, still they agree in having a changeless being, and are thus measured by aeviternity. ”