God always has been. There are several scriptures that mention this. And he always will be.
For thus says the One who is high and lifted up,The Bible uses words that have a temporal component, this is reasonable given that we dwell in time and conceiving anything outside time may well be impossible. Nonetheless, this does not necessarily imply just that time is and God has been forever. The first words of the Bible are:
who inhabits eternity, whose name is Holy: (Isaiah 57)
Blessed be the LORD, the God of Israel,
from everlasting to everlasting! (Psalm 41)
Your throne is established from of old;
you are from everlasting. (Psalm 93)
Abraham planted a tamarisk tree in Beersheba and called there on the name of the LORD, the Everlasting God. (Genesis 21)
The eternal God is your dwelling place,
and underneath are the everlasting arms. (Deuteronomy 33)
For to us a child is born,to us a son is given; and the government shall be upon his shoulder,and his name shall be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace. (Isaiah 9)
The sun shall be no more your light by day,nor for brightness shall the moon give you light; but the LORD will be your everlasting light,and your God will be your glory. (Isaiah 60)
But the LORD is the true God; he is the living God and the everlasting King. (Jeremiah 10)
But grow in the grace and knowledge of our Lord and Savior Jesus Christ. To him be the glory both now and to the day of eternity. (2 Peter 3)
In the beginning God created the heavens and the earth.This presupposes there was a beginning. A time when time itself was created. Yet God was already there.
We can call this eternity. Not just forever past to forever future but beyond time.
Whether eternity is a time dimension is unknown, but whatever qualities it has (if it is any more that just the existence of God) time is surely a subset of it. In the same way that past present and future are all subsets of all time, time itself must be a subset of eternity, as eternity does not cease when time was created and therefore time exists in it.
An analogy to space can help. 1 dimension is a subset of 2 dimensions. 2 dimensions has a infinite number of single dimensions, but no 1 dimension is a preferential reference frame. Now one can define (read create) a primary reference frame for a line but the 2 dimensions do not cease to exist. Time as it now exists may have meant little or nothing in eternity until it was defined/ created.
I am not suggesting that eternity is 2 (or 3) dimensions of time. I am not suggesting that cause and effect are not real. Rather just the idea that while eternity is different to time, it may be different in that it is more than time, that it contains it.