25 “Help me, O Lord my God: O save me according to Your mercy”. This may be referred to the whole, both to the Head and to the body: to the Head, owing to His having taken the form of a servant; to the body, on account of the servants themselves. For He might even in them have said unto God, “Help Me:” and, “O save Me:” as in them He said unto Paul, “Why do you persecute Me”? The following words, “according to Your mercy,” describe grace given gratuitously, not according to the merit of works.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)