The Islamic Sharia seeks to free people from all kinds of submission, except to Allah, the Almighty, and teaches that belief or disbelief is an affair between the worshipper and his God alone. Prophet Muhammad [PBUH] instructed us to deal with people based on what they say, and not to judge them by what they bear in their hearts. We are not allowed to scrutinize people’s hearts. No one can know what is in another’s heart but Allah. So, we judge people by their outward deeds and Allah judges their inward intentions.
The practice of the terrorist groups of labeling their opponents and those who do not adopt their bloody ideology as disbelievers has nothing to do with Islam. When doing so, they violate all Islamic teachings in this regard. Prophet Muhammad [PBUH] warned every Muslim against declaring their fellow Muslims as disbelievers, shedding their blood, seizing their properties and violating their honor.
Such terrorist groups use terminology such as kafir (infidel) murtadd (apostate) to describe those who reject their extremist ideology, and thus find a pretext for shedding their blood and destroying their houses. This definitely goes against the teachings of Islam. Prophet Muhammad (PBUH) clearly defined belief (iman) when he was asked about it by angel Jibril as “Belief is to believe in Allah, His angels, His books, His messengers, the Last Day, and to believe in God’s Preordainment both its good and its bad.” (Narrated by Al-Bukhari and Muslim). All Muslim scholars agree that belief is an inward affair, and that no one is to be declared disbeliever until she/he declares their denial to the Islamic teachings. Prophet Muhammad says, “If a man says to his brother, O Kafir (disbeliever)!' Then surely one of them is as such (i.e., a Kafir).” [Al-Bukhari 5/2264]