Limits On Our Knowledge Of Pre-Reformation History

It's common for critics of Protestantism to claim that various Protestant beliefs are absent in the historical record prior to the Reformation, were only held by a small number of people during that timeframe, etc. For documentation that those Protestant beliefs were more widespread than critics suggest, see here. But another point that should be made is that we sometimes have significantly little record of individuals and groups who plausibly, sometimes probably, held the views in question.

For example, I've written a lot over the years about the beliefs of pre-Reformation groups like the Waldensians and Lollards. Yet, it's often the case that what we know about them comes from their opponents. We're going by trial records, for instance. Think of Norman Tanner's Heresy Trials In The Diocese Of Norwich, 1428-31 (London, England: Royal Historical Society, 1977). In their trial records, the Lollards Tanner wrote about were frequently asked about certain issues: who we should pray to, purgatory, issues pertaining to the sacraments, etc. But there were other issues that were never brought up, at least in the English portions of the trial records Tanner cites. I've documented widespread opposition to and agnosticism about the Immaculate Conception and the Assumption of Mary among mainstream patristic and medieval sources, for example, even into the second millennium of church history. How likely is it that Lollards who opposed praying to Mary, opposed venerating images of her, and so on never opposed the idea that she was immaculately conceived or the idea that she was bodily assumed to heaven? But if the church officials who conducted the trials didn't ask them about those issues, and we have no or inadequate records of the beliefs of those Lollards elsewhere, then we don't have any explicit testimony from them on those subjects. We should keep in mind how incomplete our records sometimes are and how plausible it is that the beliefs in question were more widespread than we can document with explicit testimony.