Mediator Dei §8

⁸ Indeed, though we are sorely grieved to note, on the one hand, that there are places where the spirit, understanding or practice of the sacred liturgy is defective, or all but inexistent, We observe with considerable anxiety and some misgiving, that elsewhere certain enthusiasts, over-eager in their search for novelty, are straying beyond the path of sound doctrine and prudence. Not seldom, in fact, they interlard their plans and hopes for a revival of the sacred liturgy with principles which compromise this holiest of causes in theory or practice, and sometimes even taint it with errors touching Catholic faith and ascetical doctrine.