121 Composers, filled with the Christian spirit, should feel that their vocation is to cultivate sacred music and increase its store of treasures. Let them produce compositions which have the qualities proper to genuine sacred music, not confining themselves to works which can be sung only by large choirs, but providing also for the needs of small choirs and for the active participation of the entire assembly of the faithful. The texts intended to be sung must always be in conformity with Catholic doctrine; indeed they should be drawn chiefly from holy scripture and from liturgical sources.
Source: Sacrosanctum Concilium (Vatican.va)