Praise be to Allaah.
The Sahaabah, Taabi’een and the Ahl al-Sunnah who came after them were unanimously agreed that whoever says or does something which is blatantly kufr is a kaafir, without any need to show that he believes it to be permissible.
The scholars agreed that kufr may take the form of denying, disbelieving or turning away.
This may be a verbal action, such as insulting Allaah or His Messenger (peace and blessings of Allaah be upon him), or mocking the religion and its rulings; or it may be a physical action, such as prostrating to idols, circumambulating graves, or offering sacrifices to the jinn and idols.
Or it may be an act of omission, such as not doing a certain kind of action at all. Ishaaq ibn Taahawayh and others narrated that the Sahaabah were agreed that the one who does not pray deliberately is a kaafir. It was narrated in Saheeh Muslim via Ibn Jurayj from Abu’l-Zubayr al-Makki from Jaabir that the Prophet (peace and blessings of Allaah be upon him) said, “Between a man and shirk or al-kufr there stands his neglecting the prayer.”
The use of the definite article al- here indicates that major kufr (al-kufr al-akbar) is being referred to. But there are some differences among the imaams of the madhhabs concerning the kufr of one who does not pray. Some said that he does not become a kaafir in a complete sense so long as he does not deny that prayer is obligatory.
Others said that he is guilty of major kufr, because there was consensus among the Sahaabah on that point, although there were differences as to the point at which he becomes guilty of kufr by not praying. Some said that he is a kaafir is he neglects to pray one prayer until the time for it is over; others said that he is not a kaafir unless he neglects prayer completely.
To sum up, Ahl al-Sunnah do not regard a person as a kaafir for general sins or for every sin, as the Khawaarij and Mu’tazilah said when they denounced people as kaafirs for major sins. They regarded as sins some things that are not sins, and applied the rulings of kufr in such cases. Sometimes they would condemn people for the apparent meaning of their words (i.e., they were too quick to judge). This is what many of the later Khawaarij and Mu’tazilah do, because when judging people they do not differentiate between the action and the person, or between one issue and another. They may denounce as a kaafir anyone who does not agree with them in these deviations. They were described by the Messenger of Allaah (peace and blessings of Allaah be upon him), “They kill the people of Islam and leave alone the people who worship idols.” (Narrated by al-Bukhaari and Muslim from the hadeeth of Abu Sa’eed).
The Ahl al-Sunnah tread a middle path between the Khawaarij and the Murji’ah. They do not denounce those who commit major sins as kaafirs, so long as they do not believe their actions to be permissible. Neither do they agree with the view of the Murji’ah, that sin does not undermine a person’s faith at all, and that no one can be denounced as a kaafir unless he is known to believe that his action is permitted. This is false according to the Qur’aan, Sunnah and scholarly consensus (ijmaa’). So anyone who insults Allaah or His Messenger (peace and blessings of Allaah be upon him) is a kaafir, with no need to show that he believes his action to be permissible. Scholarly consensus on this point was narrated by more than one scholar. Ishaaq ibn Raahawayh said that the scholars were unanimously agreed that whoever insults Allaah, may He be glorified, or slanders His Messenger (peace and blessings of Allaah be upon him), or rejects anything that Allaah has revealed, or kills one of the Prophets of Allaah – even if he believes in that which Allaah has revealed – is a kaafir.
And Allaah knows best.