What are the "sure mercies of David"?

What are "the sure mercies of David" (Isaiah 55:3; Acts 13:34)?

The expression is reminiscent of God’s everlasting covenant with David and his household (2 Samuel 7:16; 2 Chronicles 21:7; Psalm 89:3, 4). God promised David regarding his son, "But my mercy shall not depart away from him, as I took it from Saul. . . And thine house and thy kingdom shall be established for ever before thee: thy throne shall be established for ever" (2 Samuel 7:15, 16). "My mercy will I keep for him for evermore, and my covenant shall stand fast (translated "sure" in Isaiah 55:3) with him. His seed also will I make to endure for ever, and his throne as the days of heaven" (Psalm 89:28, 29). Therefore, Isaiah 55 anticipates the future day when the Lord will encourage His earthly people to return to Him and enjoy the fulfillment of His promises to David. This kingdom of Christ, the Son of David, is assured by the resurrection, since Acts 13:34, 35 relates these "sure mercies of David" to Psalm 16:10, "Thou shalt not suffer Thine Holy One to see corruption." "The sure mercies of David" guarantee that our Lord, the risen Son of David, will have a throne in Jerusalem and an everlasting kingdom.

D. Oliver