What is meant by Christian faith? The churches of the second century asked this question, and in doing so turned to the tradition of the apostle’s teaching for clarity. This tradition was summarized in what became known as the “rule of the faith,” a pattern of teaching inherited from the apostles. Tertullian, an early Christian writer from Carthage in the second century, writes about this rule in his Prescription Against Heretics. First, we will look at Tertullian’s presentation of the rule in Prescription. Second, we will look at the rule’s role in matters of inquiry. Third, we will examine the rule’s relation to scripture and the church. Finally, we will determine the rule’s relationship to the notion of the apostolic tradition. We will ultimately show that Tertullian employs the rule of the rule in Prescription as the principle of apostolic teaching to be used against the heresies of the time.