Medieval authors commonly distinguish between two evidential situations: cases where the facts are so evident that we cannot help but assent, and cases where the evidence is not so strong, on either side, as to make belief compulsory. In these latter, non-evident cases, it is often said that we are free to choose whether or not to assent. Although most later medieval authors embraced this sort of “doxastic voluntarism” in one or another form, they were aware of its problematic nature, and developed the theory in a wide range of different ways. The project of this paper is to understand why and how medieval theories treat belief as voluntary, and to look at the implications of the theory for faith and heresy. Although we of course think of heresy as a distinctively religious, even medieval notion, I will suggest that we might do well to consider reviving the notion today, for purely secular purposes.