Are Christians obliged to pay taxes? Do they commit a mortal sin if they evade taxes? At the end of the Middle Ages, as the capacity of individual sovereigns to collect taxes increased and the ancient patrimonial concept of taxation dwindled, Catholic moral theology started to question the relationship between conscience and human laws and between theological guilt and punishment. The question of tax fraud soon became an object of debate in casuistry, which was divided into two camps: one criticised the burden and injustice of secular taxes and encouraged confessors to pardon those who dodged them; the other insisted that paying taxes and obeying civic duties were undisputable necessities. This article analyses the debate, explains its rationale and traces its history into modern times.