The formula ‘lex orandi, lex credendi’ expresses the long-acknowledged belief that the liturgy is didactic. One major concern of the present liturgical renewal is to make the liturgy more expressive of the gospel message which it proclaims. This problem is one of communication, authenticity, and meaningfulness. But another fundamental problem also warrants serious attention. What should the expressed message be? What truths ought the liturgy to communicate? The effectiveness of liturgical teaching is one problem; the propriety and value of the content of that teaching is another. It is precisely at this point that theology reverently and respectfully asks permission to judge the liturgy. Theology urges the need of an inverted formula, ‘lex credendi, lex orandi.’

Liturgy, as rite, is a pattern of sacred behaviour, and as such evolves more slowly than theology. The latter is constantly pushed onward by man’s desire to know, while liturgy is sedentary, conservative. Not that liturgy never reflects the changes in theological thinking. Just as theology has developed, so liturgy has gradually evolved. The really primitive residues in the mass, for instance, are few and short. In fact, liturgy can at times even seem to outrun theology. The unpreparedness on the part of a few professional theologians and bookish bishops at Vatican II for changes demanded by missionaries and pastors is ample proof of it. Nevertheless, liturgy ordinarily prefers calm repetitiveness, while theology at its best continually longs for new discovery and readjustment.