The Prophet’s (peace and blessings be upon him) hadith: “Whoever declares a Muslim disbeliever, he himself has become a disbeliever.” Al-Nawawi mentioned several opinions regarding it, including that it is not to be taken at face value because declaring someone a disbeliever is a sin.

Ali Hasan: A brother is asking about the saying of the Prophet (peace and blessings be upon him) . He states that Imam al-Nawawi mentioned several opinions regarding this hadith, and then stated that one who declares a Muslim disbeliever does not become a disbeliever himself. He said the reason for this is that declaring someone a disbeliever is a sin, and therefore the one who commits a sin does not become a disbeliever. Is this explanation and reasoning based on evidence? And what is your opinion on this?

The Shaykh : If this report from Imam al-Nawawi is authentic, it is not to be taken in an absolute sense. However, it is correct that not everyone who declares a Muslim disbeliever becomes a disbeliever in the sense of apostasy, meaning leaving the religion. Rather, the necessary detail is this: One who declares a Muslim disbeliever must first be a mujtahid who is distant from self-interest and advocating for it, and second, one who works according to the principles of Islamic jurisprudence, meaning he is knowledgeable of the correct path of fiqh from the Book and the Sunnah. If he declares someone a disbeliever while observing these two conditions—being distant from desire and distant from ignorance, i.e., holding fast to correct knowledge—and it turns out that the one declared a disbeliever is not as the one who declared him believed, then the one who declared him a disbeliever does not incur disbelief upon himself except in the other case: if he declared him a disbeliever not being driven away from desire and ignorance, but rather being immersed in ignorance and desire. For this is the one to whom the statement in some authentic hadiths applies. If that is the case, disbelief befalls him; otherwise, the disbelief returns to him, meaning it comes back to him. This is the detail that should be considered between the disbelief we always and forever refer to as doctrinal disbelief and practical disbelief. If one declares a Muslim disbeliever with correct ijtihad, he owes nothing and is rather rewarded. But if one declares someone a disbeliever by following desire and ignorance, then either he means actually leaving the religion when he attributes disbelief to a Muslim who believes in Allah and His Messenger, while he knows that he is truly a believer; in that case, the disbelief returns to him. Or he may have been lax in some of the conditions, in which case he is a sinner and a wrongdoer, as Imam al-Nawawi (may Allah have mercy on him) said. This is what appears to me as an answer to this question.