The opinion of the Khawarij regarding unbelief of sinners is based on the idea that works are a pillar of faith. The Salaf [the first generations of Muslims, considered an ideal example to be imitated] “among whom Malik [ibn Anas], al-Shafi‘i, Ahmad [ibn Hanbal] and Ishaq bin Rahawayh, maintained that faith comprises of belief (i‘tiqad), confession (iqrar) and works (‘amal). They believed, however, that believing is at the basis of faith, that confession is an expression and sign thereof (in the presence of which society can apply norms of faith to those who profess it), and that works are a condition for having a perfect faith. If works are not carried out, one’s faith is imperfect, but its foundation is still intact”. Ibn Hajar said: “The Salaf have stated: [to have faith means] to believe with the heart, profess with one’s tongue and act according to the Pillars [of Islam, in other words, prayer, fasting, etc.]”.
Al-Bukhari reports, relying on Abu Dharr, God be pleased by him, that the Prophet – may prayer and peace be on Him – said: “If a man accuses another man of iniquity and unbelief, these accusations will be redirected towards himself if the man he accuses is not guilty.” Thus in Islam a sinner continues to be a Muslim and cannot be excommunicated. Faith does not fail even if works fail
To this end Ibn Taymiyya said something very important: “Nobody can accuse a Muslim of unbelief, no matter how much he has sinned or erred, until proof has been shown against him. If someone declares oneself a Muslim, a doubt is not enough, real proof is needed in order to declare him guilty. God does not deny the faith of Muslims that fight each other, as His words show: “If two parties of the believers fight, put things right between them; then, if one of them is insolent against the other, fight the insolent one till it reverts to God's commandment. If it reverts, set things right between them equitably, and be just. Surely God loves the just” (49:9).