"Any group of people that rebels against a single precept of the clear and reliably transmitted prescripts of Islam has to be fought, according to the leading scholars of Islam, even if the members of the group pronounce the Islamic confession of faith. If such people make a public formal confession of their faith, but, at the same time, refuse to carry out the five daily prayers, then it is obligatory to fight them. If they refuse to pay the religious zakaat-tax, it is obligatory to fight them until they do so. Similarly, if they refuse to keep the fast of the month of Ramadan or to perform the Pilgrimage to Mecca, and similarly if they refuse to forbid abominations or adultery or gambling or wine or anything else that is forbidden by the laws of Islam. Similarly [they have to be fought and killed] if they refuse to apply the judgment of the Book and the Example of the Prophet to matters of life and property, or merchandise and commodities of any kind. Similarly [they have to be fought and killed] if they refuse to compel what is good and to prohibit what is bad, or refuse to fight the infidels until they surrender to the Muslims and humbly pay the poll tax. Similarly if they introduce innovations that are contrary to the Book and the Example of the Apostle and that are not consistent with the example of the pious forefathers, like introducing deviant opinions concerning the names of God or verses from the Koran."
Ibn Taymiyya, 14th C.