Invented with the creation and
spread of
Christianity, a heretic (in the middle-ages) is every one who did not accept or questioned the Catholic church or its laws. Unlike what the Church
spread, Heretics were rarely Satanists or non-believers.
In fact, in many cases, they had a stronger and more realist belief in God than that of the Church followers. It is just that the Heretics believed the Church's ways were wrong and corrupted, and that is why they followed their own way.