God's promise by Nathan of the perpetuity of the kingdom in David's house made an era in the progress of revelation. A new element was thereby added to devout hope, and a new object presented to faith. The prophecy of the Messiah entered upon a new stage, bearing a relation, as its successive stages always did, to the history which supplies a framework for it. Now, for the first time, He can be set forth as the king of Israel; now the width of the promise, which at first embraced the seed of the woman, and then was limited to the seed of Abraham, and thereafter to the tribe of Judah, is still further limited to the house of David. The beam is narrowed as it is focussed into greater brilliance, and the personal Messiah begins to be faintly discerned in words which are to have a partial preparatory fulfillment, in itself prophetic, in the collective Davidic monarchs whose office is itself a prophecy. This passage is the wonderful burst of praise which sprang from David's heart in answer to Nathan's words. In many of the Psalms later than this prophecy we find clear traces of that expectation of the personal Messiah, which gradually shaped itself, under divine inspiration, in David, as contained in Nathan's message. But this thanksgiving prayer, which was the immediate reflection of the astounding new message, has not yet penetrated its depth nor discovered its rich contents, but sees in it only the promise of the continuance of kingship in his descendants. We do not learn the fulness of God's gracious promises on first hearing them. Life and experience and the teaching of His Spirit are needed to enable us to count our treasure, and we are richer than we know.
This prayer is a prose psalm outside the Psalter. It consists of two parts,--a burst of astonished thanksgiving and a stream of earnest petition, grasping the divine promise and turning it into a prayer.