Summary: The covenant God made with King David, that from David’s seed a righteous King would sit on the Throne of Israel and reign over Israel forever, was specifically applied to Jesus by Gabriel.
6 For unto us a Child is born, unto us a Son is given; and the government will be upon His shoulder. And His name will be called wonderful, Counselor, Mighty God, everlasting Father, Prince of Peace.
7 Of the increase of His government and peace there will be no end, upon the throne of David and over His kingdom, to order it and establish it with judgment and justice from that time forward, even forever. The zeal of the LORD of hosts will perform this.
31 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS.
32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
The covenant God made with King David, that from David’s seed a righteous King would sit on the Throne of Israel and reign over Israel forever, was specifically applied to Jesus by Gabriel. The question is, When? In Peter’s first sermon in Acts 2, he interpreted the Davidic Covenant in such a way that leads some to conclude it is fulfilled now, by Christ’s being seated in heaven at the Father’s right hand.
Peter intended to establish Jesus’ identity as "the Christ" (Messiah - King of Israel) prophesied in the Old Testament who WILL physically reign over Israel as "King of the Jews." He was preaching to a crowd of Jews from the Diaspora who had come to Jerusalem to worship on the feast days.
22 Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know:
23 Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain:
24 Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it.
Peter launched into a series of Davidic kingly prophecies from the Psalms to prove that Jesus was the subject of their fulfillment, not David himself.
25 For David speaketh concerning him, I foresaw the Lord always before my face, for he is on my right hand, that I should not be moved:
26 Therefore did my heart rejoice, and my tongue was glad; moreover also my flesh shall rest in hope:
27 Because thou wilt not leave my soul in hell, neither wilt thou suffer thine Holy One to see corruption.
28 Thou hast made known to me the ways of life; thou shalt make me full of joy with thy countenance.
29 Men and brethren, let me freely speak unto you of the patriarch David, that he is both dead and buried, and his sepulchre is with us unto this day.
30 Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne;
31 He seeing this before spake of the resurrection of Christ, that his soul was not left in hell, neither his flesh did see corruption.