From early on, the Rule of Faith (regula fidei) designated the sum content of “apostolic” teaching, as set down in Scripture, baptismal confession and apostolic teaching patterns. As a second-century faith declaration of essential Christian teaching and interpretation, this Rule offered a framework for distinguishing between true and false belief, and – on a more basic level – provided a foundation for the faith communities to understand, practice, share, defend, refine and broaden their beliefs. Referring to the revealed core content of Christian faith, the regula fidei could be rendered as a brief faith summary about the only God and his Son, a hermeneutical structure for scriptural reading, or as a broader, missionary or catechetical presentation of the faith. Examples of the latter include Irenaeus’ Demonstration of the Apostolic Preaching and the Jerusalem church’s two-phase catechetical programme of scriptural exposition and triune creedal rendering, as noted by Egeria. The Rule could thus signify the body of teaching transmitted in the faith communities by Scripture and apostolic tradition. Emerging fixed, or semi-fixed, declaratory creeds, or symbols, such as early variants of the Apostles’ Creed (symbolum apostolorum) soon tended to be viewed as compressed, common summaries of the Rule of Faith (Augustine Sermo 213; Rufinus, Comm. in symb. apost. 2). Following some further introductory comments and a brief history of research pertaining to the regula fidei, the present paper discusses the origins, the key functions, and the basic structure of the Rule of Faith and the emerging Apostles’ Creed. Some concluding remarks and a brief comparative account of the regula fidei and the emergent symbolum apostolorum round off the discussion. The main focus of the paper is the second- and early-third-century Rule of Faith.