The "rule of faith" (Latin: regula fidei) was the early Church's concise summary of the core apostolic message — a creedal outline covering creation, the incarnation, crucifixion, resurrection, and coming judgment. It was not a fixed written text but a normative summary of what the apostles preached, passed down through the churches and formalized in baptismal confessions. Tertullian provides the clearest definition in the late second century: the rule of faith prescribes belief in one Creator God, whose Word became flesh in Jesus Christ, died, rose, and will return to judge the living and the dead. Irenaeus and Tertullian both insisted that the rule of faith — not Scripture read in isolation — was the authoritative framework within which Christian teaching must be understood. Without it, they argued, heretics could twist Scripture to say almost anything.
Tertullian defines the rule of faith as the creedal summary prescribing belief in one God who created all things through his Word; that this Word became flesh in the Virgin, preached the new law, was crucified, rose the third day, ascended to the Father, sent the Holy Spirit, and will return to judge. He declares: "This rule, as it will be proved, was taught by Christ, and raises amongst ourselves no other questions than those which heresies introduce, and which make men heretics." (ANF-03, Ch. XIII)
Irenaeus summarizes the rule of faith as the Church's one confession "received from the apostles and their disciples" — one God the Father, one Christ Jesus through whom all things were made, one Holy Spirit who spoke through the prophets. This uniform faith, held by all the churches from Jerusalem to the ends of the earth, is the measure against which deviant teaching is judged.
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