Joseph Fitzmyer SJ notes that the rule of faith (Latin: regula fidei) (where 'rule' has the sense of a measure such as a ruler) is a phrase rooted in the Apostle Paul's admonition to the Christians in Rome in the Epistle to the Romans 5:13 12:6, which says, "We have different gifts, according to the grace given us.
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.
[2] The rule of faith is the name given to the ultimate authority or standard in religious belief, such as the Word of God (Dei verbum) as contained in Scripture and Apostolic Tradition,[3] as among Catholics; theoria, as among the Eastern Orthodox; the Sola scriptura (Bible alone doctrine), as among some Protestants; the Wesleyan Quadrilateral of faith, which held that Scripture, tradition, reason, and experience, as among other Protestants; and reason alone, as among Rationalist philosophers.
In Against Heresies 1.9.4 he talks about it being received by baptism and continues, in the next chapter, to explain: …this faith: in one God, the Father Almighty, who made the heaven and the earth and the seas and all the things that are in them; and in one Christ Jesus, the Son of God, who was made flesh for our salvation; and in the Holy Spirit, who made known through the prophets the plan of salvation, and the coming, and the birth from a virgin, and the passion, and the resurrection from the dead, and the bodily ascension into heaven of the beloved Christ Jesus, our Lord, and his future appearing from heaven in the glory of the Father to sum up all things and to raise anew all flesh of the whole human race…Elsewhere, in the preface to his The Demonstration of the Apostolic Preaching, Irenaeus reiterates the need to "hold the rule of the faith without deviation.
[10] In conservative[vague] Protestantism Romans 12:6 is viewed as the biblical reference for the term "analogy of the faith" (i.e., αναλογἰα τῆς πἰστεως).