Assalaamu alaykum. It is said in a hadith that if anyone calls another person a kaafir (disbeliever), then if the person is not kaafir, the man who called him kaafir will become kaafir. On the other hand, it is said that if anyone doubts about any kaafir, then he will become kaafir. Then how should a common muslim judge who is kaafir?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

It has been proven that the Prophet said, “If a man says to his brother (i.e. his fellow Muslim): 'O kaafir (disbeliever)' and he is not as such, then it returns back to him.” [Al-Bukhari and Muslim]

In another narration, the Prophet said, “Whoever calls a (Muslim) man a kaafir or says to him ‘O enemy of Allaah’, and he is not as such, it returns back to him.” [Al-Bukhari and Muslim]

But the Prophet’s saying “it returns back to him" is interpreted by the scholars to mean minor disbelief (kufr asghar), not major disbelief that takes a Muslim out of the fold of Islam. They provided evidence of the hadeeth narrated by Thaabit ibn adh-Dhahhaak, who said, “The Prophet said, 'Cursing a Muslim is like killing him, and whoever accuses a believer of kufr (disbelief), it is like killing him.'” [Al-Bukhari]

Ibn Qudaamah said, “These ahaadeeth were said to show the seriousness of this act and to deter people from saying so and to resemble the one who says this to the non-Muslims, and it is not really meant that a person who said so becomes a non-Muslim.”

As regards doubting the disbelief of a  disbeliever, then what is meant is the one who is originally a non-Muslim from other false religions; the scholars stated that whoever does not consider a disbeliever whose disbelief is confirmed for sure, such as the Jews and Christians or people from other (false) religions, or that he doubts their disbelief, then he is a disbeliever.

Al-Qaadhi 'Iyyadh, in his book ash-Shifaa’, when mentioning what is unanimously agreed upon to be kufr, said, “It is for this reason that we consider whoever follows a religion other than Islam a non-Muslim, or whoever does not declare them [i.e. non-Muslims] as non-Muslims, or doubts about their religion (being false), or considers their religion to be true, even if he [such a person] shows Islam and believes in Islam, and believes that any other religion is void; (we consider that) he is a non-Muslim by showing what contradicts that (what he claims).”

Kashshaaf al-Qinaa', authored by Al-Buhooti from the Hanbali School of jurisprudence, reads, “Whoever does not declare the one who follows a religion other than Islam like the Christians and the Jews as a disbeliever, or doubts their disbelief, or believes that their religion is true, then he is a non-Muslim as he is denying the saying of Allaah (which means): {And whoever desires other than Islam as religion - never will it be accepted from him, and he, in the Hereafter, will be among the losers.} [Quran 3:85]”

This is in regard to the real and true non-Muslim. As regards the Muslim who commits an act that takes him out of the fold of Islam, then it is not permissible to say to him that he has committed shirk (polytheism) or kufr unless the conditions of declaring him as a disbeliever are met while there are no impediments of declaring him as such, among which is that he must be an adult, of sound mind, having free will, and not be excused due to ignorance or misinterpretation. It is only the scholars who are firmly grounded in religion who can speak about this subject; those who combine both knowledge in religion and the knowledge about how to implement its rulings in real life and who can differentiate between acts that cannot be interpreted otherwise but to mean kufr and what may involve disbelief and other (issues) and between what can be excused because of ignorance, or misinterpretation, or otherwise, and what is not likely to mean this.

However, a layman should believe that the non-Muslims who follow (or believe in) false religions are disbelievers, and the laymen and all other people know that they are disbelievers, such as the Jews, the Christians and the Magi and other people like them. As regards whoever follows the religion of Islam, then it is not permissible for a layman to declare him as a disbeliever except after the scholars rule that he is a disbeliever, after having established the evidence against him and clarified the truth to him.

Shaykh Ibn Taymiyyah said, “No one should declare a Muslim a kaafir, even if such a Muslim has done wrong and erred, until the evidence is established against him and the truth is clarified to him. Verily, he whose Islam is established with certainty cannot be declared a kaafir because of doubt (it can only be done with certainty). Rather, he cannot be called a non-Muslim except after establishing the evidence against him and removing all misconceptions that he might have."

Allaah knows best.