Charles H. Spurgeon on Psalms 89:2

I have said, etc. The perpetuity of mercy is one eminent piece of this Psalm, for with that he begins: Mercy shall be built up for ever, etc. And they are the sure mercies of our spiritual David (Christ), he means. Now, to set forth the perpetuity hereof, he first useth words that express firmitude, as established, built up for ever, Ps 89:2,4. Then he uses such similitudes as are taken from things which are held most firm and inviolable amongst men, as Ps 89:4, foedus incidi, I have cut or engraven my covenant (so in the Hebrew), alluding to what was then in use, when covenants were mutually to be made, such as they intended to be inviolate, and never to be broken; to signify so much, they did engrave and cut them into the most durable lasting matter, as marble, or brass, or the like. You may see this to have been the way of writing in use, as what was to last for ever: as Job 19:23-24. "Oh, that my words were now written! oh that they were printed in a book! That they were graven with an iron pen and lead in the rock for ever!" And what is that rock or marble here? No other than the heart itself of our gracious and most merciful Jehovah, and his most unalterable and immovable purposes, truth and faithfulness. This is that foundation in the heavens, whereon mercy is built up for ever, as Ps 89:2, which (as the Apostle says) "remains for ever"; and so they become "the sure mercies of David", Isa 60:3. Again, solemn oaths amongst men serve to ratify and make things sworn to perpetual. This also is there specified as having been taken by God: "Once have I sworn by my holiness", etc., and sworn by him that cannot lie, and sworn to that end, "to show the immutability of his counsel", Heb 6:17. And not only is the immutability of his mercy illustrated by these things taken from what is firm on earth, but he ascends up to the heavens, and first into the very highest heavens: Ps 89:2, For I have said, Mercy shall be built up for ever; thy faithfulness shalt thou establish in the very heavens: comparing them to an house built not on earth, or upon a foundation of earth, which thieves break through, and violence destroys, but in heaven, whither they cannot reach. —Thomas Goodwin.