This article proposes an interpretation of the use of the lemma ‘ὁ κανὼν τὴς ἀληθείας - regula veritatis’ in Irenaeus’ Adversus Haereses. Widespread interpretations read it as a compendium of the main items of christian faith (more or less as a symbolum) or as indicating the wholeness of the faith (as synonym of veritas), focusing themselves in terms of the “contents” of faith. We propose to understand it in a more formal way, as a lemma that indicates the formal structure of christian truth (μορφή), the specific shape that keeps the objects of faith joined together.