Short answer
Kufr means disbelief: rejecting Allah, His messengers, or something clearly known to be part of Islam. Its root meaning is to cover or deny, so it is also used for ingratitude. A person who disbelieves is called a kafir. Only qualified scholars rule that a particular Muslim has left Islam.
Major kufr takes a person out of Islam; lesser kufr, such as ingratitude for blessings or sins the hadith call kufr, does not. Hanafi jurists hold that if a Muslim's words can be read in a way that does not mean kufr, even by a weak interpretation, he is not ruled a disbeliever.
The four schools agree on this.
Explanation
The Arabic root k-f-r means to cover. A farmer who covers seed with soil was called a kafir, and the Quran uses the plural for tillers in 57:20. Disbelief is called kufr because it covers the truth a person knows or should know. The same root means ingratitude, covering up a favour: "If you are grateful, I will surely increase you; but if you deny, indeed My punishment is severe" (14:7).
Scholars divide it into two levels:
- Major kufr (kufr akbar) takes a person out of Islam. Its forms include denial, arrogant refusal like that of Iblis, doubt about Allah or the Prophet ﷺ, hypocrisy, and rejecting or mocking what is clearly part of the religion.
- Lesser kufr (kufr asghar) is a serious sin called kufr in the texts, such as ingratitude toward a spouse, or Muslims fighting each other. It does not take a person out of Islam.
"Kafir" is the Quran's word for a disbeliever, and Muslims also say non-Muslim, or for Jews and Christians, People of the Book. It is a description, but it should not be thrown around as an insult. Declaring a particular Muslim a kafir (takfir) is very serious. The Prophet ﷺ warned that if a man calls his brother "kafir", it returns to one of them. Hanafi books say a fatwa of kufr is not given when there is any way to understand a Muslim's words otherwise.
For the opposite concepts, tawheed and iman, and for shirk, read our guide to tawheed, shirk and kufr.
The evidence
Quran, Surah Ibrahim 14:7
وَإِذْ تَأَذَّنَ رَبُّكُمْ لَئِن شَكَرْتُمْ لَأَزِيدَنَّكُمْ ۖ وَلَئِن كَفَرْتُمْ إِنَّ عَذَابِى لَشَدِيدٌۭ ﴿٧﴾
And [remember] when your Lord proclaimed, 'If you are grateful, I will surely increase you [in favor]; but if you deny, indeed, My punishment is severe.' "
Uses the root of kufr for ingratitude.
Quran, Surah An-Nisaa 4:136
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ ءَامِنُوا۟ بِٱللَّهِ وَرَسُولِهِۦ وَٱلْكِتَٰبِ ٱلَّذِى نَزَّلَ عَلَىٰ رَسُولِهِۦ وَٱلْكِتَٰبِ ٱلَّذِىٓ أَنزَلَ مِن قَبْلُ ۚ وَمَن يَكْفُرْ بِٱللَّهِ وَمَلَٰٓئِكَتِهِۦ وَكُتُبِهِۦ وَرُسُلِهِۦ وَٱلْيَوْمِ ٱلْءَاخِرِ فَقَدْ ضَلَّ ضَلَٰلًۢا بَعِيدًا ﴿١٣٦﴾
O you who have believed, believe in Allah and His Messenger and the Book that He sent down upon His Messenger and the Scripture which He sent down before. And whoever disbelieves in Allah, His angels, His books, His messengers, and the Last Day has certainly gone far astray.
Defines major disbelief as rejecting the articles of faith.
Quran, Surah An-Nisaa 4:94
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ إِذَا ضَرَبْتُمْ فِى سَبِيلِ ٱللَّهِ فَتَبَيَّنُوا۟ وَلَا تَقُولُوا۟ لِمَنْ أَلْقَىٰٓ إِلَيْكُمُ ٱلسَّلَٰمَ لَسْتَ مُؤْمِنًۭا تَبْتَغُونَ عَرَضَ ٱلْحَيَوٰةِ ٱلدُّنْيَا فَعِندَ ٱللَّهِ مَغَانِمُ كَثِيرَةٌۭ ۚ كَذَٰلِكَ كُنتُم مِّن قَبْلُ فَمَنَّ ٱللَّهُ عَلَيْكُمْ فَتَبَيَّنُوٓا۟ ۚ إِنَّ ٱللَّهَ كَانَ بِمَا تَعْمَلُونَ خَبِيرًۭا ﴿٩٤﴾
O you who have believed, when you go forth [to fight] in the cause of Allah, investigate; and do not say to one who gives you [a greeting of] peace "You are not a believer," aspiring for the goods of worldly life; for with Allah are many acquisitions. You [yourselves] were like that before; then Allah conferred His favor upon you, so investigate. Indeed Allah is ever, with what you do, Acquainted.
Warns against rashly declaring people outside the faith.
Sahih al-Bukhari 6103
Sahih... إِذَا قَالَ الرَّجُلُ لأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهِ أَحَدُهُمَا ...
"If a man says to his brother, "O kafir", it returns to one of them."
Shows how serious it is to call a Muslim a disbeliever.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Jihad, Bab al-Murtadd. A Muslim is not ruled a disbeliever if his words carry a meaning other than kufr.
- al-Bahr al-Ra'iq, Bab Ahkam al-Murtaddin. Sets out the Hanafi caution in declaring a Muslim a disbeliever.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.