Calling a Muslim a Disbeliever

Imām Ibn Taimiyyah, may Allāh have mercy on him, said:

It is not [allowed] for anyone to call anyone from among Muslims a disbeliever, even if he had made a mistake and fallen into error, until proofs have been established upon him and the straight path has been made clear to him; one whose īmān¹ has been established with certainty is not to have that removed from him due to doubt; rather, it is not to be removed except after establishing the proofs [against him] and removing [all] doubts.

¹Īmān is speech, action, and beliefs; it increases with acts of obedience [to Allāh and decreases with acts of disobedience to Him (tr.).

Majmūʿ Al-Fatāwá, vol. 12, p. 501