David refers to the Lord's past actions (raising David to the throne of Israel) and the future promise (a lasting dynasty with the Messiah to come from David’s line). David says that the Lord has done these things because it is his will and it is in keeping with his word (of promise, see also Genesis 17:6; Genesis 22:17–18; Genesis 49:10).
21 Because of your promise, and according to your own heart, you have brought about all this greatness, to make your servant know it.