51 “Magnifying the salvation of His King”. God, who magnifies, so as to make wonderful, the salvation, which His Son gives to believers. “And showing mercy to His Christ:” God, who shows mercy to His Christ: “To David and to His seed for evermore:” to the Deliverer Himself strong of hand, who has overcome this world; and to them whom, as believers in the Gospel, He has begotten for evermore. What things soever are spoken in this Psalm which cannot apply to the Lord Himself personally, that is to the Head of the Church, must be referred to the Church. For whole Christ speaks here, in whom are all His members.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)