‘Before the mountains were brought forth, or ever the earth and the world were made: and from age even unto age Thou art’ (v. 2). Thou therefore who art for ever, and before we were, and before the world was, hast become our refuge ever since we turned to Thee. … But he very rightly does not say, Thou wast from ages, and unto ages Thou shalt be: but puts the verb in the present, intimating that the substance of God is altogether immutable. It is not, He was, and Shall be, but only Is. Whence the expression, I Am that I Am; and, I Am ‘hath sent me unto you;’ (Exod. iii. 14.) and, ‘Thou shalt change them, and they shall be changed: but Thou art the same, and Thy years shall not fail.’ (Ps. cii. 26, 27.) Behold then the eternity that is our refuge, that we may fly thither from the mutability of time, there to remain for evermore.
–Saint Augustine, from his Exposition on the Book of Psalms, Psalm XC (my emphasis).