1 Let us praise the Lord both in voice, and in understanding, and in good works; and, as this Psalm exhorts, let us sing unto Him a new song. It begins: “Sing ye to the Lord a new song. His praise is in the Church of the Saints”. The old man has an old song, the new man a new song. The Old Testament is an old song, the New Testament a new song. In the Old Testament are temporal and earthly promises. Whoso loves earthly things sings an old song: let him that desires to sing a new song, love the things of eternity.
Love itself is new and eternal; therefore is it ever new, because it never grows old....And this song is of peace, this song is of charity. Whoso severs himself from the union of the saints, sings not a new song; for he has followed old strife, not new charity. In new charity what is there? Peace, the bond of an holy society, a spiritual union, a building of living stones. Where is this? Not in one place, but throughout the whole world. This is said in another Psalm, “Sing unto the Lord, all the earth.” From this is understood, that he who sings not with the whole earth, sings an old song, whatever words proceed out of his mouth....We have already said, brethren, that all the earth sings a new song.
He who sings not with the whole earth a new song, let him sing what he will, let his tongue sound forth Halleluia, let him utter it all day and all night, my ears are not so much bent to hear the voice of the singer, but I seek the deeds of the doer. For I ask, and say, “What is it that you sing?” He answers, “Halleluia.” What is “Halleluia”? “Praise ye the Lord.” Come, let us praise the Lord together. If you praise the Lord, and I praise the Lord, why are we at variance? Charity praises the Lord, discord blasphemes the Lord....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)