Is it permissible to label any Muslim as a non-Muslim?
Excommunication of an individual from Islam is known as takfīr and results in the individual no longer being afforded the same rights and interactions as a Muslim. Throughout the history different dispositions and beliefs were marginalised under the pretexts of heresy and their advocates were declared as unbelievers (kuffār). However, several hadith reported on the authorities of Imams and Companions reprimand Muslims for engaging in takfīr.
This understanding is formed on the basis of the following reports:
"He who says to his brother 'O disbeliever', then it returns upon one of them."
"He who accuses a believer of kufr (disbelief) then it is like killing him."
Moreover, the sanctity of a Muslim is emphasised in both the Quran (4:93, 4:29, 49: 12) and hadith. Islam abhors slandering, lying, backbiting, or speaking ill of another Muslim. A Muslim must avoid hurting or causing harm to another fellow Muslim. In fact, loving another believer is from the traits of those who follow Islam. In essence, the dangerous practice of declaring Muslim as a non-Muslim contravenes the instruction and example of the Prophet. The prophet who displayed compassion to those who disagreed with him and overlooked their faults and adopting his characteristics allows for much needed unity amongst the Muslim ummah.
 For example, see Ṣaḥīḥ al-Bukhāri, 4: 67; 5: 110-111; al-Jaʿfariyāt, 134.
 Ṣaḥīḥ al-Bukhāri, 7: 97; al-Jaʿfariyāt, 134.
 Ṣaḥīḥ al-Bukhāri, 7: 84; al-Karājikī, Kanz al-fawāʾid, 1: 150.
 For instance, “All of the Muslim to the Muslim is inviolable; his blood, his wealth and his honour." Ṣaḥīḥ al-Muslim, 7:11; Amālī al-Murtaḍā, 1: 630.