48 “O God, who givest Me vengeance, and subduest the people under Me”. O God, who avengest Me by subduing the people under Me. “My Deliverer from My angry enemies:” the Jews crying out, “Crucify Him, Crucify Him.”
49. “From them that rise up against Me You will exalt Me”. From the Jews that rise up against Me in My passion, You will exalt Me in My resurrection. “From the unjust man You will deliver Me.” From their unjust rule You will deliver Me.
50. “For this cause will I confess to You among the Gentiles, O Lord”. For this cause shall the Gentiles confess to You through Me, O Lord. “And I will sing unto Your Name.” And You shall be more widely known by My good deeds.
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)