These qualities [sacredness, beauty, universality] are to be found, in the highest degree, in Gregorian Chant, which is, consequently, the Chant proper to the Roman Church, the only chant she has inherited from the ancient fathers, which she has jealously guarded for centuries in her liturgical codices, which she directly proposes to the faithful as her own, which she prescribes exclusively for some parts of the liturgy, and which the most recent studies have so happily restored to their integrity and purity.

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.

The ancient traditional Gregorian Chant must, therefore, in a large measure be restored to the functions of public worship, and the fact must be accepted by all that an ecclesiastical function loses none of its solemnity when accompanied by this music alone.

Special efforts are to be made to restore the use of the Gregorian Chant by the people, so that the faithful may again take a more active part in the ecclesiastical offices, as was the case in ancient times.

--St Pius X, Tra le sollecitudini paragraph 3