Terminology Tuesday: RULE OF FAITH (Regula Fidei)
One of the names used to describe outline statements of Christian belief which circulated in the 2nd-cent. Church and were designed to make clear the essential contents of the Christian faith, to serve as guides in the exegesis of Scripture (e.g. *Irenaeus, Adv. Haer. 1. 9. 4), and to distinguish the orthodox tradition from traditions to which heretics appealed. Alternative names were the ‘rule of truth’, the ‘law of faith’ or the ‘norm (κανών) of truth’. Unlike *creeds, which came later, these formularies varied in wording, though it was claimed that they faithfully reflected NT teaching, and did not differ from one another in their essential content. This content was held to have descended unchanged from apostolic times, in contrast to the spurious traditions of the heretics, which were taken to be later developments and mutually incompatible.
Gal. 6:16 is the biblical origin of this terminology. Other patristic instances include Irenaeus, Demonstration, 3 and 6, and Tertullian, De Pud. 8. 15, De Virg. Vel. 1. 4. D. van den Eynde, Les Normes de l’Enseignement Chrétien dans la littérature patristique des trois premiers siècles (1933), pp. 281–313, with full refs. J. N. D. Kelly, Early Christian Creeds (1950; 3rd edn., 1972), pp. 76–88. G. H. Tavard, Holy Writ or Holy Church , pp. 3–11. W. Beinert in L.Th.K. (3rd edn.), 8 (1999), cols. 976 f., s.v. ‘Regula Fidei’.
Cross, F. L., & Livingstone, E. A. (Eds.). (2005). In The Oxford dictionary of the Christian Church (3rd ed. rev., p. 1434). Oxford; New York: Oxford University Press.