This article addresses the writings of Bishop Reginald Pecock within the growing culture of vernacular literacy in fifteenth-century England in the wake of Arundel's Constitutions. The surviving texts from his vernacular works of lay instruction are examined in order to challenge the charges of heterodoxy that were levelled against him concerning the power and fallibility of the Church. It is argued that Pecock's most serious, yet unacknowledged, transgression was to address the laity in their mother tongue. By employing a language 'whos reulis ben not writen', Pecock was unintentionally validating the antisacerdotal Lollard discourse that he sought to counter. At the same time, the instability of English as a vehicle for expressing doctrinal subtleties afforded an opportunity for his opponents to wilfully mis-read his works in order to support their accusations.