The canon law dictum that ‘dubius in fide infidelis est’ offers a seemingly definitive statement on the place of doubt and uncertainty in medieval Catholicism. Yet where Catholic teaching was open to question, doubt was inseparable from faith, not merely as its obverse but as part of the process of achieving faithfulness – the trajectory outlined by Abelard in the twelfth century. The challenge for the Church was not that doubters lacked faith, but that having tested their doubts they might end up with the wrong faith: doubt preceded assurance, one way or the other. That problem is addressed in this essay by a broad examination of the ties between faith and doubt across the late Middle Ages (from the twelfth to the sixteenth centuries), arguing that uncertainty and doubt were almost unavoidable in medieval Catholicism. As the starting points in a process which could lead to heresy and despair, they also had a positive role in developing and securing orthodox faith.