This is a popular rule of Prosper of Aquitaine. It means the Law of Prayer is the Law of Belief. In other words, how you worship shapes what you believe. Many people are unaware of this truth. They have been told and believe worship forms are neutral.
There is no such thing as neutrality in worship form. There is no Lutheran Substance and Non-Denominational Style. No Lutheran Substance and Baptist Style. No Lutheran Substance and Pentecostal Style. Etc. By worshiping like a non-Denominational, Baptist, Pentecostal, etc., you will become one in practice & doctrine.
That's the heart of lex orandi, lex credendi: you are what you do. This is why the Divine Service is to be loved and cherished above all other forms of worship. The Divine Service catechizes the Church's children in the truth of the one, holy, catholic, and apostolic faith.