"And thine house and thy kingdom shall be established for ever before thee: thy throne shall be established for ever." (2 Samuel 7:16)
No other ordinary human being, not even the greatest of men, was ever given a promise like this promise to David. It can be understood, however, when one realizes that David is a type of Christ and that, in terms of His human genealogy, Christ did indeed inherit the right to David's throne. As the angel Gabriel told Mary: "the Lord God shall give unto him the throne of his father David: And he shall reign over the house of Jacob for ever" (Luke 1:32-33). The coming Messiah is identified as this promised Son of David in the Old Testament prophecies (e.g., Isaiah 9:6-7).
Without attempting to discuss the eschatological implications of these great prophecies, it is remarkable just to note the striking typological relation of David to Christ (and, correspondingly, of Saul to Adam). Saul, like Adam, had a wonderful physique and every natural advantage; he was given dominion over a new order of things under God; he received God's Spirit and his seed would have reigned forever had he not failed by intruding into a forbidden sphere; he was then rejected by God because of his disobedience, and finally the Spirit of God departed from him.
David, however, is a beautiful type of the second Adam, the Lord Jesus Christ. Though anointed, he was not accepted by his brethren; he was a shepherd and performed great services for his people before becoming king; but he was rejected and condemned to death. God delivered him, but even then he was only accepted by a few, until suddenly all Israel accepted him and he was promised an eternal kingdom. Christ now claims: "I am the root and the offspring of David"--that is, both Creator and heir of David--"and the bright and morning star" (Revelation 22:16). HMM