2 Sam 7:12,16, 2 Sam 23:5, Acts 2:29-32,36
In the Old Testament, God promised David that He would send an eternal covenant, that is, Christ who would come as David's descendant. (Isa 55:3, 2 Samuel 7:12,16, 2 Samuel 23:5)
Isa 55:3 Incline your ear, and come unto me: hear, and your soul shall live; and I will make an everlasting covenant with you, even the sure mercies of David. 4 Behold, I have given him for a witness to the people, a leader and commander to the people.
2 Sam 7:12 And when thy days be fulfilled, and thou shalt sleep with thy fathers, I will set up thy seed after thee, which shall proceed out of thy bowels, and I will establish his kingdom. 16 And thine house and thy kingdom shall be established for ever before thee: thy throne shall be established for ever.
2 Sam 23:5 Although my house be not so with God; yet he hath made with me an everlasting covenant, ordered in all things, and sure: for this is all my salvation, and all my desire, although he make it not to grow.
God sent Christ, the eternal promise promised to David in the Old Testament. God also raised Christ from the dead according to the sure grace he had granted David. That Christ is Jesus. (Acts 2:29-32,36)
Acts 2:29 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. 32 This Jesus hath God raised up, whereof we all are witnesses. 36 Therefore let all the house of Israel know assuredly, that God hath made the same Jesus, whom ye have crucified, both Lord and Christ.