{ And if they break their oaths after having made their covenant, and defame your religion, then fight the leaders of disbelief – for indeed they have no oaths – that perhaps they might cease } Al-tawba 12

The Most High is saying that if the Idolaters, those with whom you made covenant for a specific period, break their oaths, their covenants, or their agreements, { and defame your religion } , that is, slander it and undermine it. From this it can be taken that whoever slanders the Messenger (may the prayers and peace of Allah be upon him), or whoever defames the religion of Islam, is to be killed.