REGULA FIDEI ("RULE OF FAITH"): A term used so frequently in early Christian literature from the last quarter of the second century that an understanding of it is necessary to a correct idea of the religious conceptions of that period. Different forms with more or less the same meaning occur. Ho kanon tēs alētheias ("canon of truth"), regula veritatis (rule of truth), probably the oldest form was used apparently by Dionysius of Corinth (c. 160), then by Irenæus, Clement of Alexandria, Hippolytus, Tertullian, and Novatian; ho kanon tēs pisteôs, regula fidei, by Polycrates of Ephesus, Clement of Alexandria, Tertullian, and by the later Latin writers. The equivalent use of these two expressions is important for the determination of the original significance attached to them. The truth itself is the standard by which teaching and practise are to be judged (cf. Irenæus, Hr., II., xxviii. 1; ANF, i. 399). It is presupposed that this truth takes for the Christian community a definite, tangible form, such as the law was for the Jews (Rom. ii. 20), in a body of doctrine not merely held and taught by the Church, but clearly formulated. Besides the expressions already discussed, another is worth mentioning, found only in Greek writers and the versions from them--ho ekklesiastikos kanōn or ho kanōn tēs ekklēsias (Clement of Alexandria and Origen).
The ante-Nicene church never considered as the Rule of Faith the Bible or any part of it. Certain expressions of recent writers show that it is not unnecessary to point out that the word kanon, with or without qualifying additions, is never used until after Eusebius to designate the Bible, and that even after the word had begun to be applied to the collection or Scriptural books, the sense mentioned above is never given to it by the Greeks. This is explained by the fact that the early Church used this word for something else--the baptismal formula. It is quite evident that in the oldest and most explicit witnesses for the use of the word, Irenæus and Tertullian, this was known primarily as the rule of faith. When the former (I., ix. 4) says "he who retains unchangeable in his heart the rule of the truth which he received by means of baptism," the expression "rule of truth" can not mean any sum total of truths as to which instruction has been conveyed before or after baptism, but only a formula which the neophyte has made his own by a profession of faith made at the time of baptism. This was "the faith," which the convert received from the teaching Church and was to keep as the standard for his subsequent life and for the testing of all doctrines presented to him. With Tertullian the regula fidei is identical with the sacramentum fidei, the rule of faith with that which he so often designates as the oath of allegiance of the soldiers of Christ (Ad martyras, iii.). The prevalent view in both these authors is the same as that expressed by Augustine when he says to the catechumens at the traditio symboli, "receive, sons, the rule of faith which is called 'the symbol'" (Serm., ccxiii.; Serm. i., ad catechumenos de symbolo). That similar expressions are occasionally used of the Nicene creed shows at least that the Rule of Faith was a formulated confession, and thus that in the ante-Nicene period it could not refer to anything but the baptismal creed, the only one then existing. In a word, the early Fathers considered Christ himself as the giver of the Rule, though they admitted freely that its actual words were an expansion of the nucleus recorded in the Gospels, regarding it as only a development of the baptismal formula; and, on the other hand, the whole body of teaching current in the undisputed Catholic Church was to them but an expansion of the creed, and thus the term "Rule of Faith" could be, as it is occasionally found, applied to this whole body.