11 “An hymn to all His Saints.” Know ye what an hymn is? It is a song with praise of God. If you praise God and singest not, you utter no hymn: if you sing and praisest not God, you utter no hymn: if you praise anything else, which pertains not to the praise of God, although you sing and praisest, you utter no hymn. An hymn then contains these three things, song, and praise, and that of God. Praise then of God in song is called an hymn. What then means, “An hymn to all His Saints”? Let His Saints receive an hymn: let His Saints utter an hymn: for this is what they are to receive in the end, an everlasting hymn....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)