The rule of faith (Latin: regula fidei) 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 12:6, 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." (N.I.V.). The last phrase, "in proportion to his faith" is in Greek 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.:
Let our "seeking," therefore be in that which is our own, and from those who are our own, and concerning that which is our own, - that, and only that, which can become an object of inquiry without impairing the rule of faith.