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']" (Daniel 12:3). Now if God alone were eternal, there could not be many eternities. Therefore God alone is not the only eternal.
Objection 2. Further, it is written "Depart, ye cursed into eternal [Douay: 'everlasting'] fire" (Matthew 25:41). Therefore God is not the only eternal.
Objection 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 (Question 9, Article 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" (Ecclesiastes 1:4). Again, some things are called eternal in Scripture because of the length of their duration, although they are in nature corruptible; thus (Psalm 75:5) the hills are called "eternal" and we read "of the fruits of the eternal hills." (Deuteronomy 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 to Objection 1. There are said to be many eternities, accordingly as many share in eternity, by the contemplation of God.
Reply to Objection 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" (Psalm 80:16).
Reply to Objection 3. Necessary means a certain mode of truth; and truth, according to the Philosopher (Metaph. vi), is in the mind. Therefore in this sense the true and necessary are eternal, because they are in the eternal mind, which is the divine intellect alone; hence it does not follow that anything beside God is eternal.
Source: Summa Theologica (New Advent)