Thomas Aquinas - Summa Theologica, Part I (Prima Pars)

Further, those things are said to be measured by time which have a beginning and an end in time, because in everything which is moved there is a beginning, and there is an end. But as whatever is wholly immutable can have no succession, so it has no beginning, and no end.

Thus eternity is known from two sources: first, because what is eternal is interminable—that is, has no beginning nor end (that is, no term either way); secondly, because eternity has no succession, being simultaneously whole.

Reply Obj. 1: Simple things are usually defined by way of negation; as "a point is that which has no parts." 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 Obj. 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 Obj. 3: Eternity is called whole, not because it has parts, but because it is wanting in nothing.

Reply Obj. 4: As God, although incorporeal, is named in Scripture metaphorically by corporeal names, so eternity though simultaneously whole, is called by names implying time and succession.

Reply Obj. 5: Two things are to be considered in time: time itself, which is successive; and the "now" of time, which is imperfect. Hence the expression "simultaneously-whole" is used to remove the idea of time, and the word "perfect" is used to exclude the "now" of time.

Reply Obj. 6: Whatever is possessed, is held firmly and quietly; therefore to designate the immutability and permanence of eternity, we use the word "possession." _______________________

SECOND ARTICLE [I, Q. 10, 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.

Obj. 2: Further, what is before eternity, and after eternity, is not measured by eternity. But, as Aristotle says (De Causis), "God is before eternity and He is after eternity": for it is written that "the Lord shall reign for eternity, and beyond [*Douay: 'for ever and ever']" (Ex. 15:18). Therefore to be eternal does not belong to God.

Obj. 3: Further, eternity is a kind of measure. But to be measured belongs not to God. Therefore it does not belong to Him to be eternal.

Obj. 4: Further, in eternity, there is no present, past or future, since it is simultaneously whole; as was said in the preceding article. But words denoting present, past and future time are applied to God in Scripture. Therefore God is not eternal.

On the contrary, Athanasius says in his Creed: "The Father is eternal, the Son is eternal, the Holy Ghost is eternal."

I answer that, The idea of eternity follows immutability, as the idea of time follows movement, as appears from the preceding article. Hence, as God is supremely immutable, it supremely belongs to Him to be eternal. Nor is He eternal only; but He is His own eternity; whereas, no other being is its own duration, as no other is its own being. Now God is His own uniform being; and hence as He is His own essence, so He is His own eternity.

Reply Obj. 1: The "now" that stands still, is said to make eternity according to our apprehension. As the apprehension of time is caused in us by the fact that we apprehend the flow of the "now," so the apprehension of eternity is caused in us by our apprehending the "now" standing still. When Augustine says that "God is the author of eternity," this is to be understood of participated eternity. For God communicates His eternity to some in the same way as He communicates His immutability.

Reply Obj. 2: From this appears the answer to the Second Objection. For God is said to be before eternity, according as it is shared by immaterial substances. Hence, also, in the same book, it is said that "intelligence is equal to eternity." In the words of Exodus, "The Lord shall reign for eternity, and beyond," eternity stands for age, as another rendering has it. 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.

Reply Obj. 3: Eternity is nothing else but God Himself. Hence God is not called eternal, as if He were in any way measured; but the idea of measurement is there taken according to the apprehension of our mind alone.

Reply Obj. 4: Words denoting different times are applied to God, because His eternity includes all times; not as if He Himself were altered through present, past and future. _______________________

THIRD ARTICLE [I, Q. 10, Art. 3]

Whether to Be Eternal Belongs to God Alone?

Objection 1: It seems that it does not belong to God alone to be eternal. For it is written that "those who instruct many to justice," shall be "as stars unto perpetual eternities [*Douay: 'for all eternity']" (Dan. 12:3). Now if God alone were eternal, there could not be many eternities. Therefore God alone is not the only eternal.

Obj. 2: Further, it is written "Depart, ye cursed into eternal [Douay: 'everlasting'] fire" (Matt. 25:41). Therefore God is not the only eternal.

Obj. 3: Further, every necessary thing is eternal. But there are many necessary things; as, for instance, all principles of demonstration and all demonstrative propositions. Therefore God is not the only eternal.

On the contrary, Jerome says (Ep. ad Damasum, xv) that "God is the only one who has no beginning." Now whatever has a beginning, is not eternal. Therefore God is the only one eternal.

I answer that, Eternity truly and properly so called is in God alone, because eternity follows on immutability; as appears from the first article. But God alone is altogether immutable, as was shown above (Q. 9, A. 1). Accordingly, however, as some receive immutability from Him, they share in His eternity. Thus some receive immutability from God in the way of never ceasing to exist; in that sense it is said of the earth, "it standeth for ever" (Eccl. 1:4). Again, some things are called eternal in Scripture because of the length of their duration, although they are in nature corruptible; thus (Ps. 75:5) the hills are called "eternal" and we read "of the fruits of the eternal hills." (Deut. 33:15). Some again, share more fully than others in the nature of eternity, inasmuch as they possess unchangeableness either in being or further still in operation; like the angels, and the blessed, who enjoy the Word, because "as regards that vision of the Word, no changing thoughts exist in the Saints," as Augustine says (De Trin. xv). Hence those who see God are said to have eternal life; according to that text, "This is eternal life, that they may know Thee the only true God," etc. (John 17:3).

Reply Obj. 1: There are said to be many eternities, accordingly as many share in eternity, by the contemplation of God.

Reply Obj. 2: The fire of hell is called eternal, only because it never ends. Still, there is change in the pains of the lost, according to the words "To extreme heat they will pass from snowy waters" (Job 24:19). Hence in hell true eternity does not exist, but rather time; according to the text of the Psalm "Their time will be for ever" (Ps. 80:16).