Rorate Caeli

Lex orandi, lex credendi

From an interview with Msgr. James P. Moroney on the new translation of the 1970 / 2002 Missal:
Msgr. Moroney, can you explain the Latin principle, Lex orandi, lex credendi?

Both the Roman Missal and the instruction Liturgiam authenticam tell us that the Roman Rite is perhaps best defined by the rites and prayers of the Sacred Liturgy. This is a simple application of the ancient principle lex orandi, lex credendi, or, the practice of our prayer is the practice of our belief. How we pray best defines what we believe. This is one of the reasons why an accurate translation of liturgical texts is so essential to the life of the Church. We will never have a clear idea of what we believe until we have a clear idea of the texts we have prayed in the Sacred Liturgy for over a millennium
From the context of the interview it seems that the monsignor is implying that the texts of the Novus Ordo represent the millenial liturgy of the Roman Church -- a strange claim that is rather widespread. Nevertheless, the idea in the highlighted passage is, in itself, correct. Here's hoping that those who read it will realize its implications...