Wikipedia
The rule of faith or analogy of faith (analogia fidei) is a phrase rooted in the Apostle Paul's admonition to the Christians in Rome in the Epistle to the Romans , which says, "We have different gifts, according to the grace given us. If a man's gift is prophesying, let him use it in proportion to his faith." (NIV, 1984) The last phrase, "in proportion to his faith" is in Greek κατὰ ἀναλογίαν τῆς πίστεως ("analogy of faith"). In Romans 12:6 this refers to one of three possible ideas: the body of Christian teachings, the person's belief and response to the grace of God, or to the type of faith that can move mountains. This phrase in Romans 12 becomes the root for later usage of the term by such Early Christian writers as Tertullian. Tertullian links it to the core set of Christian teachings, i.e.:
Usage examples of "rule of faith".
The gospel was ostentatiously displayed in the centre, but the rule of faith was defined by the Papal and Imperial ministers, who moderated the thirteen sessions of the council of Chalcedon.