Shaykh al-Islam Ibn Taymiyyah warning to those who accuse others of disbelief is as if he is responding to the advocates of takfeer from among the extremists of our day who accuse of blasphemy all and sundry and all those who did not agree with their opinion, he said: “The renegade Kharijites who the Prophet (PBUH) ordered to fight. And the leader of the faithful Ali ibn Abi Talib, one of the caliphs agreed to fight them along with all the imams of Islam and the companions and followers, and those that came after them. Yet, Ali ibn Abi Talib and Saad ibn Abi Waqas and other companions did not accuse them of being non-Muslims, and fought them as Muslims. Ali ibn Abi Talib did not fight them until they shed innocent blood and raided the wealth of the Muslims. He fought them to put an end to their oppression and tyranny, not because they are disbelievers. If this group whose deviance has been established by scripture and consensus were not considered disbelievers yet were fought, then what about those different denominations who are uncertain about the truth due to the mistakes of those more learned than them. (Fatawa)