The rule lex orandi lex credendi [the rule of prayer is the rule of belief… in other words, what one practices shapes one’s belief and vice versa] appears to be the way things go even more so with false doctrines. It was along the path of liturgical development that the worst heresies made their way into the church. Consider the doctrines which came in the with the cult [worship] of saints and of Mary. In Luke 1:28, the Ave Maria is the greeting which the angel gives Mary. This greeting from heaven acclaims her as the mother of our Redeemer, the mother of God. Thus she is confessed also in evangelical [Lutheran] doctrine, for her Son is no less than truly God. When this greeting from heaven becomes an invocation of her on earth, we may observe the cult of Mary. Even though liturgy may indeed have its inherent laws of essence and form, yet it does not produce its doctrinal content out of itself. This it receives from what God has revealed. Whatever the liturgy says is subject always to the judgment of the norma normans [ “ultimate/norming norm”] of Holy Scripture. The assertion that something could be liturgically right and doctrinally wrong may indeed be true of the ancient mystery religions, but never of the Christian liturgy.
– Hermann Sasse, “The Lord’s Supper in the Catholic Mass” (1941), The Lonely Way, Volume II, p. 25