59 The Church is without question a living organism, and as an organism, in respect of the sacred liturgy also, she grows, matures, develops, adapts and accommodates herself to temporal needs and circumstances, provided only that the integrity of her doctrine be safeguarded. This notwithstanding, the temerity and daring of those who introduce novel liturgical practices, or call for the revival of obsolete rites out of harmony with prevailing laws and rubrics, deserve severe reproof. It has pained Us grievously to note, Venerable Brethren, that such innovations are actually being introduced, not merely in minor details but in matters of major importance as well. We instance, in point of fact, those who make use of the vernacular in the celebration of the august eucharistic sacrifice; those who transfer certain feast-days - which have been appointed and established after mature deliberation - to other dates; those, finally, who delete from the prayerbooks approved for public use the sacred texts of the Old Testament, deeming them little suited and inopportune for modern times.
Source: Mediator Dei (Vatican.va)