Albert Barnes Commentary Psalms 90:2

"Before the mountains were brought forth, Or ever thou hadst formed the earth and the world, Even from everlasting to everlasting, thou art God." — Psalms 90:2 (ASV)

Before the mountains were brought forth - Before the earth brought forth or produced the mountains. In the description of the creation, it would be natural to represent the mountains as the first objects that appeared, emerging from the waters, and therefore as the “first” or “most ancient” of created objects. The phrase, therefore, is equivalent to saying, before the earth was created. The literal meaning of the expression “were brought forth” is, in Hebrew, “were born.” The mountains are mentioned as the most ancient things in creation in (Deuteronomy 33:15). Compare (Genesis 49:26); (Habakkuk 3:6).

Or ever thou hadst formed - literally, “had brought forth.” Compare (Job 39:1).

The earth and the world - The word “earth” here is used to denote the world as distinguished either from heaven (Genesis 1:1) or from the sea (Genesis 1:10). The term “world” in the original is commonly used to denote the earth considered as “inhabited,” or as capable of being inhabited—a dwelling place for living beings.

Even from everlasting to everlasting - From duration stretching backward without limit to duration stretching forward without limit; that is, from eternal ages to eternal ages, or forever.

Thou art God - Or, “You, O God.” The idea is that he was always and will always be God: the God, the true God, the only God, the unchangeable God. At any period in the past—during the existence of the earth or the heavens, or before either was formed—he existed, with all the attributes essential to Deity; at any period in the future—during the existence of the earth and the heavens, or beyond, as far as the mind can reach into the future, and even beyond that—he will still exist unchanged, with all the attributes of Deity. The creation of the universe made no change in him; its destruction would not vary the mode of his existence or make him in any respect a different being. There could not be a more absolute and unambiguous declaration, as there could not be one more sublime, of the eternity of God.

The mind cannot take in a grander thought than that there is one eternal and immutable Being.