The Deviation of the Khawarij
The unauthorized takfir of a Muslim, that is, declaring him a non-Muslim without a legally valid reason according to Islamic law, is considered in Islamic doctrine a reprehensible and rejected innovation (bid'ah, pl. bida') introduced into Islam by consensus.
A well-known historical example of this can be found in the various factions of the so-called khawarij sect, who carried out takfir on Muslims due to the commission of major sins (kaba'ir, sing. kabirah).
This erroneous idea of takfir based on major sins (at-takfiru bi-l-kaba'ir) forms the fundamental principle of all splinter groups of the so-called khawarij. Today, the most well-known group adhering to this belief is the one that has been referred to in recent decades as jama'atu t-takfiri wa-l-hijrah. Another major religious group that adheres to this idea is the so-called 'ibadiyyah, which, for example, is practically the state religion in present-day Oman, although it treats this concept merely as a highly theoretical doctrinal belief.