33. In the middle ages, the relationship between Liturgy and popular piety is constant and complex, but a dual movement can be detected in that same relationship: the Liturgy inspired and nourished various expressions of popular piety; and several forms of popular piety were assumed by, and integrated into the Liturgy. This is especially true with regard to the rites of consecration of persons, the assumption of personal obligations, the dedication of places, the institution of feasts and to the various blessings.
There were storm clouds, however. Note the developments that led to the Reformation, and a council that was too late to solve the problem of a fractured Christendom. Can the sins of the 16th century be attributed in part to the divorce between good theology and catechesis and the devotional life of believers?
A dualism, however, prevailed between Liturgy and popular piety. Towards the end of the middles ages, both, however, went through a period of crisis. Because of the collapse of cultic unity, secondary elements in the Liturgy acquired an excessive relevance to the detriment of its central elements. In popular piety, because of the lack of adequate catechesis, deviations and exaggerations threatened the correct expressions of Christian worship.
The full document, the Directory on Popular Piety and the Liturgy, is online at the Vatican site.