Lex credendi legem statuat supplicandi

In seminary formation we were often quoted an ambiguous phrase regarding the relationship between faith and worship: lex orandi => lex credendi, which may be erroneously interpreted to mean that the way we pray teaches us what we are to believe. That is not necessarily true because our prayer itself might be heretical if it is expressing or representing false doctrine! It is much clearer to say that our faith, hope and charity are the source of our forms of prayer.

This important distinction regarding the necessary credal foundation of the liturgy was clarified by Pope Pius XII in his encyclical Mediator Dei in which he said that "if one desires to differentiate and describe the relationship between faith and the sacred liturgy in absolute and general terms, it is perfectly correct to say, 'Lex credendi legem statuat supplicandi' – let the rule of belief determine the rule of prayer. The same holds true for the other theological virtues also, “In . . . fide, spe, caritate continuato desiderio semper oramus” – we pray always, with constant yearning in faith, hope and charity." (Mediator Dei 46-48)
N.B. Mediator Dei is an essential document for understanding the nature of the Catholic priesthood in relationship to the laity, Papal authority and the liturgy. The document is an eloquent explanation of the Church's (in each of her various types of members) proper disposition in Her worship and the very specific nature of that Catholic worship.