Shaykh Attiatu’Allah Al-Libi said in an anecdote about the Khawarij:
Some (Khawarij) who repented and were guided by Allah – after spending a long period with them – once told us that there were cases where they would doubt Allah and his Prophet – peace be upon him – and his message and the Qur’an! Yes by Allah, they were so extreme and stern in their religion to the extent where they weren’t pleased with Allah the Almighty’s mighty mercy, nor did they accept Allah’s favours. They continue to be extreme on themselves untill they get destroyed.
Thus, Shaykh Abu Muhammad Al-Maqdisi – who is an expert on them and indeed familiar with their ways of deviance – he gave his Al-Thalāthinyāh book another title called “Al-Jifr treatise – excessiveness in Takfir leads to Kufr”.
(“Jawāb Suaāl Fi Jihad A’Daf” p.4-6).
From the practical applications (of this extremism) is what Abu Maryam Al-azdi narrated:
“Some dear brothers of Khorasān (who are now in Syria) told me that a group made Takfir on the Angels because they prostrated to Adam and they deemed Iblis to be a monotheist because he refrained from prostrating to other than Allah. This was in the province of Anķwarāda in Afghanistan”.
(The Neo-Kharajites, p.28).
These extremists deemed Muslim he whom Allah declared to be a Kafir, and deemed Kafir those whom Allah decreed to be believers. This belief lead them to Kufr because they rejected clear cut revelation.