Directory on Popular Piety and the Liturgy (DPPL) gives a brief history:
6. The history of the Western Church is marked by the flowering among the Christian people of multiple and varied expressions of simple and fervent faith in God, of love for Christ the Redeemer, of invocations of the Holy Spirit, of devotion to the Blessed Virgin Mary, of the veneration of the Saints of commitment to conversion and of fraternal charity. These expressions have grown up alongside the Liturgy. Treatment of this vast and complex material which is sometimes referred to as “popular religiosity” or “popular piety”* lacks a uniform terminology. Hence it will be necessary to adopt a certain precision of language. Without pretending to resolve all difficulties in this area, it will be useful to outline the commonly understood meaning of certain terms employed in this document.
* Treating of the same material, the Apostolic Exhortation, Evangelii Nuntiandi, 48, for example, having recalled its richness, states: “because of this we gladly call it popular piety, that is, religion of the people, rather than religiosity of the people”; the Apostolic Exhortation Catechesi tradendae, 54, uses the expression “popular piety”; the Code of Canon Law, can. 1234.1, adopts the term “popular piety”; John Paul II uses the term “popular piety” in the Apostolic Letter Vicesimus quintus annus; The Catechism of the Catholic Church, 1674-1676, uses the expression “popular religiosity” but is also aware of the term “popular piety” (1679); the fourth Instruction for the correct implementation of the conciliar Constitution on the Sacred Liturgy (37-40) Varietates legitimae, published by the Congregation for Divine Worship and the Discipline of the Sacraments (January 1994) employs the expression “popular piety” in article 45.
This extended footnote gives some of the post-conciliar background of the terminology the curia would like to use. Note the wide range of source material: not only liturgy, but catechesis and evangelization. One would be correct to say this has far-reaching impact on many areas of Catholic life. When it comes to specifics, we’ll get into more detail as we progress through the document.