25 “Bless ye the Lord, all you His hosts: ye servants of His that do His pleasure”. All you angels, all you that are mighty in strength: ye that do His word: all you His hosts, you servants of His that do His pleasure, do ye, you bless the Lord. For all they who live wickedly, though their tongues be silent, by their lips do curse the Lord. What does it profit if your tongue sings a hymn, while your life breathes sacrilege? By living ill you have set many tongues to blasphemy. Your tongue is given to the hymn, the tongues of those who behold you, to blasphemy. If then thou dost wish to bless the Lord, do His word, do His will....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)