I have, al-Hamdulillah, learned a lot through your web site. I wanted to ask about al-Khawarej, did Ali Ibn Abi say they were Kuffar or not?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

al-Khawarej is a deviant group that strayed from the true religion. This group adopts a number of fundamentals that contradict what Ahl al-Sunnah Wal-Jama'ah believe in. In other words, al-Khawarej believe that a person who commits a major sin is not a Muslim. In addition they speak evil of some companions of the Prophet (Sallallahu Alaihi wa Sallam) and even believe that some companions are not Muslims. al-Khawarej do not believe that believers will see Allah, The Great and Almighty, in the Hereafter.
They also deny the stoning punishment in Islam and so on.
Thus, there is disagreement among scholars concerning this deviant group. The majority of Muslim scholars do not consider them as disbelievers. It is also reported from the Emir of the faithful Ali (Radiya Allahu Anhu) that al-Khawarej are not disbelievers.
Allah knows best.