On these grounds Gregorian Chant has always been regarded as the supreme model for sacred music, so that it is fully legitimate to lay down the following rule: the more closely a composition for church approaches in its movement, inspiration and savor the Gregorian form, the more sacred and liturgical it becomes; and the more out of harmony it is with that supreme model, the less worthy it is of the temple.

Pope Pius X, in his 1903 Motu Proprio.