50 The sacred liturgy does, in fact, include divine as well as human elements. The former, instituted as they have been by God, cannot be changed in any way by men. But the human components admit of various modifications, as the needs of the age, circumstance and the good of souls may require, and as the ecclesiastical hierarchy, under guidance of the Holy Spirit, may have authorized. This will explain the marvelous variety of Eastern and Western rites. Here is the reason for the gradual addition, through successive development, of particular religious customs and practices of piety only faintly discernible in earlier times. Hence likewise it happens from time to time that certain devotions long since forgotten are revived and practiced anew. All these developments attest the abiding life of the immaculate Spouse of Jesus Christ through these many centuries. They are the sacred language she uses, as the ages run their course, to profess to her divine Spouse her own faith along with that of the nations committed to her charge, and her own unfailing love. They furnish proof, besides, of the wisdom of the teaching method she employs to arouse and nourish constantly the "Christian instinct."
Source: Mediator Dei (Vatican.va)