We might be in the best position today to understand why theologians, thinkers and leaders in the late antique and medieval period cared so much about heresy. And that is not necessarily because of our scholarship and historical knowledge of that period and of those mentalities, but because we may be starting to be confronted with it again. I’m obviously not talking about heresy in the classic sense of theological heterodoxy, but of heresy as a sociological phenomenon, with all the implications it had for the cultures of the past which took it seriously.
Heresy is mere opinion until it is called out as heresy by someone else. So the onus is on the detractor to describe it as such. This is the reason why most of the knowledge we have about medieval heresies (to single out one heresy-rich age among many) comes from those who wrote and acted against them. The voices of the heretics are drowned in the sea of indictment. In control of the historical record are those who stamp out heresy, the winners in the fight for orthodoxy, the bien pensants of the prevailing view.
Modernists as many of us are draw lines between old and new phenomena based on inherited labels and are reluctant to recognise kinship when that goes against established modes of thought. One of these modes is that religion is only to be found in, err, religion (as defined by modernist theory) and heresy only in traditional religious systems. Heresy, according to the modern understanding, can only occur within a religious framework, like Christian and Muslim heresies and heretics. But on closer examination, the reality of heresy can exist in non-religious environments, at least not religious in the traditional sense. Holding views which are considered unacceptable or in contradiction to values and ideas perceived to be self-evident or universally held prompts certain groups to deploy the same mechanisms which were used in past ages to deal with religious heresy : suppression, cancelling, intimidation, pressure to recant or offer public penance.
One can only wonder whether Joan of Arc would have tweeted her way out of the stake. A recantation to all her followers and local Church leaders may have saved her.