[The author describes how the medieval tradition answered the question of whether one can legitimately speak of collective ecclesial sin. Using principally Bonaventure as a focal point, he examines how the notion of ecclesial sin functioned simultaneously as reform rhetoric and an ecclesial apologetic of humility. Finally, he applies Bonaventure’s analysis of ecclesial sin to the present crisis regarding sexual abuse of minors to show how this idea can function even today to exhort believers to maintain unity as they struggle for reform.]