46 On this subject We judge it Our duty to rectify an attitude with which you are doubtless familiar, Venerable Brethren. We refer to the error and fallacious reasoning of those who have claimed that the sacred liturgy is a kind of proving ground for the truths to be held of faith, meaning by this that the Church is obliged to declare such a doctrine sound when it is found to have produced fruits of piety and sanctity through the sacred rites of the liturgy, and to reject it otherwise. Hence the epigram, " Lex orandi, lex credendi " - the law for prayer is the law for faith.
Source: Mediator Dei (Vatican.va)