5 “Sing to the Lord, O you saints of His.” The prophet seeing these future things, rejoices, and says, “Sing to the Lord, O you saints of His. And make confession of the remembrance of His holiness”. And make confession to Him, that He has not forgotten the sanctification, wherewith He has sanctified you, although all this intermediate period belong to your desires.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)