21 “Upon Him shall My sanctification flourish”. Upon whom? Upon Mine Anointed. For when He says, “Mine anointed,” it is the voice of the Father, who says, “I will bless her widow with blessings, and will satisfy her poor with bread. I will clothe her priests with salvation, and her saints shall rejoice and sing.” He who says, “There will I raise up the horn of David,” is God. He Himself says, “I have ordained a lantern for Mine Anointed,” because Christ is both ours and the Father's: He is our Christ, when He saves us and rules us, as He is also our Lord: He is the Son of the Father, but both our Christ and the Father's.
For if He were not the Father's Christ, it would not be said above, “For Your servant David's sake, turn not Thou away the presence of Your Anointed.” “Upon Him shall My sanctification flourish.” It flourishes upon Christ. Let none of men assume this to himself, that he himself sanctifies: otherwise it will not be true, “Upon Him shall My sanctification flourish.” The glory of sanctification shall flourish. The sanctification of Christ therefore in Christ Himself, is the power of the sanctification of God in Christ.
In that he says, “shall flourish,” he refers to His glory: for when trees flourish, then are they beautiful. Sanctification therefore is in Baptism: thence it flourishes, and is brightened. Why has the world yielded to this beauty? Because it flourishes in Christ; for, put it in man's power, and how does it then flourish? Since “all flesh in grass, and all the goodliness thereof as the flower of the grass.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)