Why is the Jesus they crucified still their king?
God swore an oath to David that He would raise up Christ from David’s line to sit on David’s throne. David, being a prophet, foretold the resurrection. Although they crucified and killed Him, God raised Him from the dead; therefore the promised King remains their king.
Les explains that Peter is quoting David in the Psalms, and David was not speaking of himself, because David was dead and buried and his sepulcher was still with them. Rather, David was a prophet and knew God had sworn with an oath that through the fruit of his loins according to the flesh—the line of the house of David—God would raise up Christ to sit on His throne. Since the one who sits on a throne is a king, Peter’s point is that the king they crucified and killed was raised from the dead by God and can still be their king. David, seeing this before, spoke of the resurrection of Christ.