The catechumenate emerged in the late second century during the period when Christianity was transitioning from a loose collection of school churches to a more unified monepiscopate. Irenaeus’s writings bear witness to the aesthetic character of knowledge in early Christian catechetical teaching during this time. For Irenaeus, the rule of truth serves as a pedagogical tool enabling new Christians to perceive the unity of creation and Scripture. This chapter looks at catechetical terminology and appeals to the Rule of Truth in Aduersus haereses and the Demonstratio.