The people of knowledge (i.e. the scholars) are of the view that it is obligated on the Muslims in every era that if they are certain about the presence of this filthy doctrine (i.e. the doctrine of the khawaarij), they should remedy it through calling to Allaah (i.e. the correct way) and enlighten the people about that. If they (i.e. khawaarij) do not obey (and come out fighting), then they are fought (by the Muslim ruler) in order to repel their harm.
[لمحة عن الفرق الضالة–page 42]