Theologians Karl Rahner and Herbert Vorgrimler noted in their dictionary of theology that there was a similar statement recorded in the Council of Ephesus (431) when it was cataloguing authoritative statements from popes. It has lex supplicandi, lex credendi, which might be rendered "the law of supplication, the law of believing" (Denzinger, 139).
Rahner and Vorgrimler explained that the statement from the Council of Ephesus developed into the theological principle that "the liturgy is the norm of faith, a witness to the infallible belief of the praying church."
If you have stayed with me through these three paragraphs, please follow me into the Roman Missal (Third Edition) and to the second eucharistic prayer.
The new translation of that prayer includes the petition, "Have mercy on us, O Lord, that...we may merit to be coheirs to eternal life..."
It is the term "merit" that makes me question what it is we believe.
If we keep praying that we may merit eternal life, then does that not influence what we believe about grace, about eternal life as pure gift?
It is Catholic theology that no one merits heaven; it is a gift.
We follow God's commands, we suffer along with Christ, not so that we will merit eternal life but because it has already been offered to us. Doing things and being good in order to earn heaven is putting the cart before the horse.
Some children are taught that they should be good in order to receive presents at Christmas. Adults are urged to be good because Christ has already offered them.
The notion that we have to do things in order to merit God's love and eternal life is reflected in the older son in Jesus' parable of the prodigal. He thought he should have been rewarded for his loyalty and service, and was upset that his younger, profligate brother was being welcomed home with a party.
Perhaps the second eucharistic prayer would be better translated "that we may inherit eternal life."
If we understood the prayer in this way, we are simply affirming that the law of believing is the law of praying, or lex credendi, lex orandi.