Charles H. Spurgeon on Psalms 89:3

I have made a covenant with my chosen, I have sworn unto David my servant. This was the ground of the Psalmist's confidence in God's mercy and truth, for he knew that the Lord had made a covenant of grace with David and his seed, and confirmed it by an oath. Here he quotes the very words of God, which were revealed to him by the Holy Spirit, and are a condensation of the original covenant in 2Sa 7:1-29. Well might he write in the former verse, "I have said", when he knew that Jehovah had said, "I have sworn." David was the Lord's elect, and with him a covenant was made, which ran along in the line of his seed until it received a final and never ending fulfilment in "the Son of David." David's house must be royal: as long as there was a sceptre in Judah, David's seed must be the only rightful dynasty; the great "King of the Jews" died with that title above his head in the three current languages of the then known world, and at this day he is owned as king by men of every tongue. The oath sworn to David has not been broken, though the temporal crown is no longer worn, for in the covenant itself his kingdom was spoken of as enduring for ever. In Christ Jesus there is a covenant established with all the Lord's chosen, and they are by grace led to be the Lord's servants, and then are ordained kings and priests by Christ Jesus. How sweet it is to see the Lord, not only making a covenant, but owning to it in after days, and bearing witness to his own oath; this ought to be solid ground for faith, and Ethan, the Ezrahite, evidently thought it so. Let the reader and writer both pause over such glorious lines, and sing of the mercies of the Lord, who thus avows the bonds of the covenant, and, in so doing, gives a renewed pledge of his faithfulness to it. "I have", says the Lord, and yet again "I have", as though he himself was nothing loath to dwell upon the theme. We also would lovingly linger over the ipsissima verba of the covenant made with David, reading them carefully and with joy. There are thus recorded in 2Sa 7:12-16: "And when thy days be fulfilled, and thou shall sleep with thy fathers, I will set up thy seed after thee, which shall proceed out of thy bowels, and I will establish his kingdom. He shall build an house for my name, and I will stablish the throne of his kingdom for ever. I will be his father, and he shall be my son. If he commit iniquity, I will chasten him with the rod of men, and with the stripes of the children of men: But my mercy shall not depart away from him, as I took it from Saul, whom I put away before thee. And thine house and thy kingdom shall be established for ever before thee: thy throne shall be established for ever." After reading this, let us remember that the Lord has said to us by his servant Isaiah, "I will make an everlasting covenant with you, even the sure mercies of David."