Ruling by KUFR is KUFR! – Ibn Taymiyyah

Sheikh Al-Islam, Ahmad Ibn Taymiyyah, Rahimahullah
Majmoo’ al Fatawa, 28/524:
“And it is known by necessity from the deen of the Muslims, and the agreement of all the Muslims, that whoever permits the following of a deen other than Islam or following a Shari’ah other than the Shari’ah of Muhammad (SALLAAHU ‘alahi wa sallam), then he is a kafir, and it is like the kufr of one who believes in part of the Book and disbelieves in part of the Book, as ALLAAH said (the meaning of which is):
«Indeed those who disbelieve in Allaah and His Messengers, and wish to distinguish between Allaah and His Messengers, and say we believe in some and disbelieve in some, and wish to take a way between that, all those are the kafiroon in truth, and WE have prepared for the kafireen a humiliating punishment. » An-Nisa’ 150-151 “
Majmoo’ al Fatawa : 28/544-545-546:
“Fighting the Tartars, those who came to the land of Shaam is wajib according to the Book and the Sunnah, for indeed ALLAAH said in the Qur’an (the meaning of which is) :
«And fight them until fitnah is no more, and the deen becomes all for ALLAAH» Al-Anfal:39.
And the deen means obedience, so if some of the deen belongs to ALLAAH and some to other than ALLAAH, fighting becomes wajib, until the deen becomes all for ALLAAH, and thus ALLAAH said (the meaning of which is):
«O You who believe, fear ALLAAH, and leave off what remains of riba (usury) if you are believers; if you do not do so, then take note of war from ALLAAH and HIS Messenger. » Al-Baqarah 278-279.
And this ayah was sent down concerning the people of Taa’if when they entered into Islam, and adhered to salah, siyam, but they refused to leave off riba, so ALLAAH declared that they are warriors against ALLAAH and HIS Messenger.
Then if it was wajib to make jihad against all these warriors against ALLAAH and HIS Messenger, what then of one who leaves many of the laws of Islam or most of them, like the Tartars ? And the ‘ulema of the Muslims have agreed that when a refusing faction (Taa’ifah Mumtani’ah) refuses some of the obligations of Islam openly without any doubt, then it is wajib to fight it; if they profess the Shahadatayn and refuse salah, or zakah, or siyam in the month of RamaDaan, or Hajj to the ancient house, or judging between them with the Book and the Sunnah, or prohibiting indecencies, or drinking, or marriage with close relatives (maHaarim), or make lives and wealth permissible without justice or riba (usury) or gambling, or jihad against the kuffaar, or imposing jizyah on the people of the book, and similar to that from the laws of Islam, then they are to be fought for that until the deen becomes all for ALLAAH.
And it is established in the two Saheehs when Omar debated with Abu Bakr about those who refused zakah, Abu Bakr said to him: ‘How can I not fight one who leaves the rights which ALLAAH and His Messenger have made obligatory, like zakah, even if they have accepted Islam ‘ And he said to him: ‘Indeed zakah is from its rights; by ALLAAH if they refuse me an inaq (rope for tying a camel) which they used give to the Messenger of ALLAAH (SALLAAHU ‘alaihi wa sallam) I will fight them for witholding it.’
Omar said: ‘Then there was none other except that I saw ALLAAH had expanded the breast of Abu Bakr to fight, and I knew that it was the truth.’
And it is established in the SaHeeH from more than one source that the Prophet (SALLAAHU ‘alaihi wa sallam) mentioned the khawarij, and said about them: ‘One of you would look down at his salah compared to their salah, and his siyam with their siyam, and his recitation with their recitation; they will recite the Qur’an but it will not travel beyond their throats, they will pass through Islam as an arrow passes through the game; wherever you find them, fight them, for indeed in their killing will be a reward from the ALLAAH for whoever kills them, on the day of Judgement. If I were to catch them, I would kill them like the killing of ‘Aad.’
And the salaf and Imams are in agreement upon the fighting of all these, and the first of those who fought them was ‘Ali bin Abi Taalib (Radi ALLAAHU ‘anhu), nor did the Muslims cease fighting them in the early period of the khilafah of Bani Umayyah and Bani ‘Abbas, even though with oppressive amirs- Hajjaj and his type were amongst those who fought them- thus all the Imams of the Muslims have ordered fighting them. And the Tartars and those similar to them, are greater in their departure from the Shari’ah of Islam than those who refused zakah, and the khawarij from the people of Taa’if, those who refused to leave off riba.
So whoever doubts in fighting them, then he is the most ignorant of people about the deen of Islam, and whenever it becomes wajib to fight them, fight them, even if they have with them those who have been forced, in accordance with the Muslims.”