The Second Vatican Council stated emphatically that “choirs must be diligently promoted while ensuring that the whole body of the faithful may be able to contribute that active participation which is rightly theirs.” The choir must not minimize the musical participation of the faithful. The congregation commonly sings unison melodies, which are more suitable for generally unrehearsed community singing. This is the primary song of the Liturgy.
Choirs and ensembles, on the other hand, comprise persons drawn from the community who possess the requisite musical skills and a commitment to the established schedule of rehearsals and Liturgies. Thus, they are able to enrich the celebration by adding musical elements beyond the capabilities of the congregation alone.
The choir may draw on the treasury of sacred music, singing compositions by composers of various periods and in various musical styles, as well as music that expresses the faith of the various cultures that enrich the Church. The music of the choir must always be appropriate to the Liturgy, either by being a proper liturgical text or by expressing themes appropriate to the Liturgy.
Sing to the Lord: Music in Divine Worship, §28-30, Issued by USCCB, November 14, 2007