The variety of early Christian groups and related questions regarding “orthodoxy” and “heresy” are the focus of a course I am offering this year (course outline here). The traditional view of “orthodoxy-first-and-heretical-deviations-later” which I’ve discussed in connection with Eusebius rests, in part, on lack of attention to followers of Jesus who were opposed by certain authors in the earliest period. Opponents in Paul’s letters (our earliest evidence starting about 50 CE) and other subsequent early Christian writings can provide an important window into opinions and practices that existed among Jesus followers from the beginning. I’ve chosen literature pertaining to the region of Asia Minor as a geographical focus for our attempt to plot out these opinions and practices, to map out the forms of Christianity in one area.
Yet there are serious (methodological) difficulties in getting back to the views and activities of such opponents. For one, the sources we have about them are quite hostile towards the worldviews and practices of such opponents, and ancient authors did not hesitate to engage in exaggeration, labeling, and name-calling. They expressed their opinions in a rhetorically-charged way, and it is likely that, if we had writings from the opponents, they may well have done the same. For an example of such counter-attacks, see my Peter vs. Simon Magus (alias Paul) in the Pseudo-Clementines (NT Apocrypha 17).
The modern historian must remain above this, so to speak, and avoid uncritically taking the position of the ancient author who condemns some other individual or group. Instead, we want to do the best we can to sift rhetorically-charged material for particularly significant items. We want to evaluate what levels of probability there are that a given practice or view goes back to the opponents in question, placing such things within broader cultural contexts. While the ancient author was interested in dispelling the opponents’ position, we want to recover it, despite the difficulties involved in doing this.
Added to this are the dangers of “mirror-reading” by which I and other scholars mean the process of mentally holding up a mirror to the literature. Here one must be careful not to assume that every point rhetorically attacked by an author necessarily has a basis in the reality of some group’s activities. There is not always a direct correspondence between one author’s proscription (condemnation) of certain behaviours and views and the actual situation at a particular locale, and we should not assume that everything condemned was actually done. Jerry Sumney has spent a good degree of his time researching precisely the difficulties in evaluating opponents in literature.