Before the mountains were brought forth, or ever thou hadst formed the earth and the world, even from everlasting to everlasting, thou art God. Ps. 90:2
The word eternity is easily pronounced but hardly understood. This is due in part to man’s frailty of nature, as a creature bound to time. Eternity, being in conflict with time, is an attribute of God that largely exceeds man’s mind. Eternity is perpetual duration having neither beginning nor end; time has both. Eternity and time differ in much the same way as the sea and rivers: the sea never changes place and is always one water, but the rivers glide along and are swallowed up by the sea. Such is time in relation to eternity.
This difficulty in understanding eternity is increased because the term is used to describe things that are only partially perpetual and not properly eternal. Eternity can be used of something having a long duration but possessing an end (Gen. 17:8; Lev. 6:20; Deut. 15:17) or of something having no end though having a beginning—such as angels and souls. Nonetheless, when eternity is used of God; it means something further; as the Scriptures attest “even from everlasting to everlasting, thou art God” (Ps. 90:2). Therefore eternity in this sense refers to the duration of God’s essence. When God is called eternal, all possibility of beginning and ending—any flux and change—is excluded. Consequently, the eternity of God is best understood negatively, as a denial that God has any measure of beginning, end or succession.
God is without beginning as the everlasting God (Gen. 21:33; Rom. 16:26; cf.Gen. 1:1; Dan.7:9). This is necessary according to God’s existence and status as Creator; for, if God does exist, and He has not received His being from another, then He must exist from eternity.
God is without end. This aspect refers to immortality, which is spoken of in Scripture more frequently than the other aspects of God’s eternity. He shall endure forever (Ps. 9:7; James 1:17; Rev. 4:9-10). His years are numberless (Job 36:26-27). This is evident by the name He gives Himself (Ex. 3:14) and the fact that He is life in His own essence (Dan 6:26; John 5:26; cf. Acts 17:28; 1 Tim. 6:16).
God is without succession. He is always the same (Ps.1 02:27; Heb. 1:10-12) and has no new progression of quantities or qualities in Himself. Of a creature, it may be said that “he was,” “he is, “or “he will be,” but of God it can only be said that “He is.” There is no increase in His knowledge (Acts 15:18) or fluctuation in His decrees (Eph. 1:4). There is no abrogation of any of His attributes. Furthermore, if God were not eternal, all His other attributes would be maimed beyond recognition. God would not be immutable (cf. Job 37:23; Mal. 3:6), infinitely perfect (cf. Job 11:7; Ps. 41:13) omnipotent (cf. Isa. 2:22; Rev. 1:8), or the first cause of all.
The eternity of God holds a word for both the unbeliever and the Christian. For the former, God’s eternity is a terror. What a folly and boldness there is in sin, since an eternal God is offended thereby! All sin is aggravated by God’s eternity. The blackness of the pagan idolatry was in exchanging the glory of the incorruptible God for things contrary to His immortal nature (Rom 1:23). It is dreadful to lie under the stroke of this eternal God, who is the “living God, and an everlasting king…the nations shall not be able to abide his indignation” (Jer.10:10). God’s eternity makes His punishment more dreadful than His power alone; His power makes it sharp, but His eternity renders it perpetual—ever to endure is the sting at the end of every lash.
But for the Christian the reality that God “remains forever” (Lam 5:19-20) is the fountain of comfort. Peace is found in fellowship with the ever merciful, good, wise, and faithful God. His eternity governs His covenant with His people—thereby He swears by Himself (Heb 6:13,16,17; Rev. 14:6 cf. Rev 4:3), and so the believer may proclaim, “This God is our God for ever and ever” (Ps. 48:14) and “Thou hast been our dwelling place in all generations” (Ps.90:1; cf. Gen.49:26). Moreover, the eternity of God ensures that the enjoyment of God in heaven will be as fresh and glorious after many ages as it was at first.
Copied from The Reformation Heritage KJV Study Bible which they —adapted from Stephen Charnock, “A Discourse upon the Eternity of God”, in The Existence and Attributes of God