In Islam, the case of belief and unbelief is a personal issue between man and God. Our prophet Muhammad (Peace be upon him) taught us to deal with people according to what they utter and not to judge the case of their hearts, that is not to set up an Inquisition to discover the believer and the disbeliever. It is Allah the Almighty who is the only one to know what is in people’s hearts. We judge people according to what is apparent to us, however their inner belief is left to Allah, as He is the only One Who calls people to account.
As for the ideology of the terrorist groups that labels their opponents disbelievers is indeed an excessive transgression. They are violating all the clear teachings of Islam in this regard. The Prophet (peace be upon him) warned that a Muslim should not declare his brother unbeliever or transgress against his soul, property and honor.
These takfiri groups use the term Riddah (apostasy) to describe the overwhelming majority of Muslims, who disagree with their extremist views, to end up with a conclusion of killing them and destroying their homes! From where did they get their teachings? The Prophet (peace be upon him) defined faith in the authentic hadith when Jabril (peace upon him) came and asked him: "faith is to believe in Allah, His Angels, His Messengers, His Books, the Last day, and the Divine Decree (Qadar), both the good of it and the bad of it." (Rported by al-Bukhari and Muslim). The whole Muslim nation unanimously agrees that faith is an act of the heart; thus nothing brings person out of belief except when denying the principles that made him a believer. The prophet (peace be upon him) said: "Whoever says to his brother 'O disbeliever' then it will be true about one of them."
(Rported by al-Bukhari)