Sebastian Castellio presented to French-speaking Protestants a vision of Christianity fundamentally different from that of Calvin. His vision was based on a belief in the opacity of Scripture and thus the temporary, provisional nature of any claims to religious truth. This need for doubt in Christianity led Castellio to his famous opposition to religious persecution and to his praise of reason as the ultimate arbiter in questions of religious truth. Castellio’s opposition to religious persecution emerged most strongly in his criticism of the execution of Michael Servetus, but he continued for the rest of his career to fight with Calvin and Theodore Beza over that issue, as well as others, such as biblical interpretation, predestination, and justification. Unlike recent studies that have downplayed Castellio’s role as a forerunner of liberal Protestantism, this book argues that he should, in fact, be viewed as such.