The eternality of God is one of the foundational beliefs of Christianity. If God is not eternal, then eternal life is not His to give to anyone. Also if God had a beginning He too would have to have a creator, at which point he is not the one true God. Without the eternality of God, Christianity is futile.
Baker's Evangelical Dictionary of Biblical Theology says this is reference to God being eternal:
God is eternal ( Deut 33:27 ; Psalm 10:16 ; 48:14 ). Scripture does not provide philosophical reflection on this fact but assumes it. The Lord is the Rock eternal ( Isa 26:4 ) and the eternal King ( Jer 10:10 ). God's word, rooted in his being and will, is likewise eternal ( Psalm 119:89 ), as are his righteous laws (119:60), his ways ( Hab 3:6 ), and his kingdom or dominion ( Daniel 4:3 Daniel 4:34 ). Since God is eternal, so are his love ( 1 Kings 10:9 ), his blessings ( Psalm 21:6 ), and all his other attributes and benefits. They endure without end; as long as God exists, so do they.
The subject of the eternality of God requires that we understand that God is infinite in relation to time. He is without beginning, and He is without ending. Eternality means not only that God dwells in unending duration from eternity past to eternity future, it means that He dwells in another dimension outside of time. Consider with me that to God, all history appears as both past and present.
Notice with me an incredible passage: Revelation 13:8.
All who dwell on the earth will worship him, everyone whose name has not been written from the foundation of the world in the book of life of the Lamb who has been slain.
How incredible it is to consider that even before the foundation of the world, the names were written in the book of life. Clear evidence that God sees everything past, present and future.
Another passage we must consider is Romans 8:30.
and these whom He predestined, He also called; and these whom He called, He also justified; and these whom He justified, He also glorified.
Notice the unbroken chain- and the fact that God sees the finished product when He looks at His children!
Why is this critical such a critical doctrine? Because as we understand the God who was, who is, and who will always be, we can draw strength and comfort in knowing that our Heavenly Father is completely sovereign and holds all things together yesterday, today and forever!
1 Thessalonians 4:16 For the Lord Himself will descend from heaven with a shout, with the voice of the archangel and with the trumpet of God, and the dead in Christ will rise first.
17 Then we who are alive and remain will be caught up together with them in the clouds to meet the Lord in the air, and so we shall always be with the Lord.
How wonderful it is that those who have trusted Christ will ALWAYS (eternally) be with the Lord!