This dissertation re-examines trials for heresy in England from 1382, which saw the first major action directed at the Wycliffite heresy in Oxford, and the early Reformation period, with an emphasis on abjurations, the oaths renouncing heretical beliefs that suspects were required to swear after their interrogations were concluded. It draws a direct link between the customs that developed around the ceremony of abjuration and the exceptionally low rate of execution for “relapsed” and “obstinate” heretics in England, compared to other major European anti-heresy campaigns of the period. Several cases are analyzed in which heretics who should have been executed, according to the letter and intention of canon law on the subject, were permitted to abjure, sometimes repeatedly. Cases that ended in execution despite intense efforts by the presiding bishop to obtain a similarly law-bending abjuration are also discussed. These cases are situated within explorations of the constitutions governing heresy trials, contrasting their use of apparently standard legal terminologies with more aggressive continental inquisitors, as well as the theology and cultural standing of oaths within both Wycliffism and the broader Late Medieval and Early Modern world. This dissertation will trace how Lollard heretics gradually accepted the necessity of false abjuration as one of a number of measures to preserve their lives and their movement, and how early adopters using coded writing carefully persuaded their co-religionists of this necessity. Furthermore, it will argue that the bishops who conducted the trial system deliberately constructed it to encourage this type of perjury, even suppressing attempts to alter heretics’ actual convictions, for the sake of social order and stability.