In the sixteenth and early seventeenth centuries, Protestant reforms and the subsequent Counter-Reformation drastically altered the religious landscape of Western Europe, eventually leading to the European wars of religion. During that time, the Protestant movement was widely equivocated with the notion of heresy. In Brittany, Protestantism never quite reached the heights that it would in e.g. mainland France: however, religious texts of the period readily mention heresy and heretics after 1625, and conversely, what it means to be a ‘good Christian’. This paper will explore a number of Middle Breton catechisms, and otherwise relevant religious didactic texts to attempt (1) a definition of heresi and heretic in Middle Breton, and how these definitions relate to that of the notion of infidel, (2) how the depiction differs in the contemporary literature and in the Catechisms and (3) why Protestantism only began to appear in Catechism in 1625 and not before.