John Calvin 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. Moses intends to set forth some high and hidden mystery, and yet he seems to speak feebly, and, as it were, in a childish manner. For who does not know that God existed before the world? We grant this is a truth that all people admit; but we will scarcely find one in a hundred who is thoroughly persuaded that God remains unchangeably the same.

God is here contrasted with created beings, who, as all know, are subject to continual changes, so that there is nothing stable under heaven. Since, in a particular way, nothing is more full of change than human life, He is here presented in a state of settled and undisturbed tranquility, so that people may not judge the nature of God by their own fluctuating condition.

Thus the eternity of which Moses speaks relates not only to the essence of God but also to His providence, by which He governs the world. Although He subjects the world to many changes, He remains unmoved; and this is true not only in regard to Himself, but also in regard to the faithful, who find from experience that, instead of being wavering, He is steadfast in His power, truth, righteousness, and goodness, just as He has been from the beginning.

This eternal and unchangeable steadfastness of God could not be perceived before the creation of the world, since there were not yet any eyes to be witnesses of it. But it may be gathered a posteriori; for while all things are subject to revolution and constant change, His nature always remains the same.

There may also be a contrast here between Him and all the false gods of the pagans, who have, little by little, crept into the world in such vast numbers through the error and folly of people. But I have already shown the objective that Moses has in view, which is that we are mistaken if we measure God by our own understanding, and that we must rise above the earth, yes, even above heaven itself, whenever we think about Him.