Charles H. Spurgeon on Psalms 118:2

Let Israel now say, that his mercy endureth for ever. God had made a covenant with their forefathers, a covenant of mercy and love, and to that covenant he was faithful evermore. Israel sinned in Egypt, provoked the Lord in the wilderness, went astray again and again under the judges, and transgressed at all times; and yet the Lord continued to regard them as his people, to favour them with his oracles, and to forgive their sins. He speedily ceased from the chastisements which they so richly deserved, because he had a favour towards them. He put his rod away the moment they repented, because his heart was full of compassion. "His mercy endureth for ever" was Israel's national hymn, which, as a people, they had been called upon to sing upon many former occasions; and now their leader, who had at last gained the place for which Jehovah had destined him, calls upon the whole nation to join with him in extolling, in this particular instance of the divine goodness, the eternal mercy of the Lord. David's success was mercy to Israel, as well as mercy to himself. If Israel does not sing, who will? If Israel does not sing of mercy, who can? If Israel does not sing when the Son of David ascends the throne, the very stones will cry out.