What is the doctrine of the Khawarij, and when did it begin?

Question:
One of the brothers asks: about the doctrine of the Khawarij, who is the leader of this sect, when did it spread, and did any of the Caliphs oppose them?

Answer:
The Khawarij are those who rebel against the obedience of the ruler of the Muslims, they break the bond of obedience, and they fight the Muslims. They declare a Muslim a disbeliever for a sin that is less than shirk (associating partners with Allah); they declare him a disbeliever for a major sin that is less than shirk. Thus, they combine two crimes: the crime of declaring disbelievers for major sins less than shirk, and the crime of breaking the bond of obedience and dividing the community. And a third crime, which is killing Muslims. The Prophet (peace and blessings be upon him) informed us that the Khawarij will fight the people of faith and call upon the people of idols. This is their habit in every age; you will find their tyranny over the Muslims and over Muslim societies. The first to fight them was the Commander of the Faithful, 'Ali ibn Abi Talib (may Allah be pleased with him). Then the Caliphs and rulers after him continued to fight them. The Prophet (peace and blessings be upon him) said about them: "Whenever a generation of them appears, it is cut off." And praise be to Allah, He makes it easy for those who extinguish their fitnah (trial, discord or temptation). Yes.