"Whenever someone calls his brother Muslim a kafir, one of them must be a kafir [either the one being accurately being called a kafir, or the one who falls into kufr, by inaccurately accusing his brother of being a kafir]." ~ Prophet Muhammad, God's peace and blessings be upon him
"To deem a thousand disbelievers Muslim is safer with God than to deem one Muslim a disbeliever." ~ Imam Abu Hanifah
"Takfir should be reserved for one who clearly falls into apostasy, states it openly, chooses it as his din, rejects the testimony of faith, and leaves the religion of Islam altogether." ~ Taqi al-Din al-Subki
"To deem a thousand disbelievers Muslim is safer with God than to deem one Muslim a disbeliever." ~ Imam Abu Hanifah
"Takfir should be reserved for one who clearly falls into apostasy, states it openly, chooses it as his din, rejects the testimony of faith, and leaves the religion of Islam altogether." ~ Taqi al-Din al-Subki
