What did God swear to David about Christ and his throne?
God swore with an oath to David that through the fruit of his loins—according to the flesh, through the house of David—He would raise up Christ to sit on David's throne. Because one who sits on a throne is a king, the crucified Christ was raised from the dead to still be their king.
Les explains that Peter, speaking of David, notes David was both dead and buried, so David was not speaking of himself. David was a prophet, and God had sworn with an oath to him that of the fruit of his loins, according to the flesh—that is, through the line of the house of David—He would raise up Christ to sit on his throne. Since a throne belongs to a king, Peter's point is that the king they crucified and killed was raised from the dead and can still be their king. David, seeing this before, spoke of the resurrection of Christ.
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;