The majority of jurists do not hold Khawaarij to be disbelievers
16-2-2015 | IslamWeb
Question:
As-salaamu 3laikum, I am confused about something I have heard. On your website it says that the Khawarij are not disbelievers. Somebody told me that they are and that the first person to be classed as from the Khawarij was a man who said to Rasulullah (salla Allahu 3laihi wa salam): 'This is not justice' about a decision that rasulullah (salla Allahu 3laihi wa salam) made. My first thought was that this must have put him out of Islam as it is criticism of a prophet. So did this incident happen and what are we to understand from it? Jazaakum Alllahu khairan
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The view that the Khawaarij are not disbelievers is the opinion of the majority of the jurists. Some scholars even went as far as to say that a scholarly consensus has been reached about this. However, what is correct is that scholars have held different opinions about that matter. The preferred scholarly view in this regard is that of the majority of the jurists, which we mentioned above.
Accusing the Prophet, sallallaahu ‘alayhi wa sallam, of injustice, as reported in the authentic Hadeeth cited in the Saheehayn of Al-Bukhari and Muslim, is undoubtedly a plain act of disbelief. Shaykh Al-Islam Ibn Taymiyyah 
said in his book As-Saarim Al-Maslool: "In brief, the words in this regard can be classified into three categories: one of them is words of disbelief, such as 'This is an unjust division that was not done for the sake of Allaah.'"
However, the scholars make a distinction between the statement and the sayer, and between the action and the doer. A person may utter words of disbelief or do an act that constitutes disbelief yet he is not to be declared a disbeliever based on the excuse of ignorance, potential misinterpretation, or coercion. For further information about the conditions of Takfeer (declaring a person or a group to be disbelievers), please refer to Fataawa 8106 and 87963.
The conditions and principles of Takfeer set by the scholars are implied from this Hadeeth and similar texts.
Allaah Knows best.