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.