His mercy endures forever

From the additional notes in Charles Spurgeon's The Treasury of David, ,Psalm 107:

Verse 1. His mercy endureth for ever. St. Paul assures us, that the covenant of grace, which is the fountain of all mercy, was made before the foundation of the world, and this he repeats in several of his epistles. The Psalmist teaches the same doctrine, and frequently calls upon us to thank God, because his mercy is for ever and ever—because his mercy is everlasting—and in the text, because "his mercy endureth for ever; "the word "endureth" is inserted by the translators, for there is no verb in the original neither in strictness of speech could there be any; because there was no time when this mercy was not exercised, neither will there be any time when the exercise of it will fail. It was begun before all worlds, when the covenant of grace was made, and it will continue to the ages of eternity, after this world is destroyed. So that mercy was, and is, and will be, "for ever", and sinful miserable man may always find relief in this eternal mercy, whenever the sense of his misery disposes him to seek for it. And does not this motive loudly call upon us to "give thanks"? Because there is mercy with God—mercy to pity the miserable—and even to relieve them—although they do not deserve it: for mercy is all free grace and unmerited love. Oh! How adorable, then, and gracious is this attribute! How sweet is it and full of consolation to the guilty.—William Romaine (1714-1795), in "A Practical Comment on the Hundred and Seventh Psalm."