The linguistic meaning of kufr is concealment or the act of covering something. Its legalistic meaning is the opposite of faith, which is lack of belief in Allah and His Messenger. Kufr does not necessarily coincide with a denial of the divine message; as some other factors, including doubt or skepticism, deliberate avoidance of the truth resulting from envy, pride, and submission to certain misleading impulses, may also be responsible for unbelief.
Kufr is divided into major and minor. The former will lead a person to become permanently incarcerated in the Hellfire, while the latter will subject the person to a transitory and not permanent punishment in the Fire.
In regard to major kufr, it can be divided into five:
- Disbelief as a result of denial. This means lack of belief in the truthfulness of Allah’s Messengers. Anyone who belies their message, either overtly or covertly, is an unbeliever. Allah says,
“Who is more unjust than one who invents a lie about Allah or denies the truth when it has come to him? Is there not in Hell a sufficient residence for the disbelievers?” (Q, 29:68)
- Disbelief through rejection and arrogance. This is when a person arrogantly and stubbornly refuses to abide by the rule of a Messenger of Allah or comply with his order, in spite of being convinced of his truthfulness and the genuineness of his mission. The proof of this in the Book of Allah is the following,
“When We said to the angels, ‘Prostrate before Adam,’ so they prostrated, except for Iblīs. He refused and was arrogant and became of the disbelievers.” (Q, 2:34)
- Skeptical unbelief. That is when one remains unconvinced of the truthfulness of Allah’s Messengers. It is also called kufr al-ẓann (disbelief to suspicion), implying thereby lack of certainty or being indecisive. The proof comes in the following verse,
“He entered his garden while he was doing wrong to himself. He said, ‘I do not think that this will ever perish, and I do not think the Hour will occur. Even if I am brought back to my Lord, I will surely find better than this as a return.’ His companion said to him, while he was conversing with him, ‘Have you disbelieved in He who created you from dust and then from a sperm-drop and then proportioned you [as] a man? As for me, I believe that Allah is my Lord, and I do not associate anyone with my Lord.’” (Q, 18:35-8)
- Disbelieve as a result of deliberate avoidance of the truth. That is to keep oneself completely away from the religion of Islam by turning a deaf ear to the divine message and paying no attention whatsoever to it. Allah says,
“But those who disbelieve, they turn away from that of which they are warned.” (Q, 46:3)
- Hypocritical unbelief or insincerity in matters of faith. That is in the case of someone who pretends to be a believer, while he is really an infidel. Allah says,
“That is because they declared faith (in Islam apparently), then disbelieved (secretly). Therefore, a seal has been set on their hearts, and thus they do not understand.” (Q, 63:3)
Hypocrisy, itself if of two types. The first is hypocrisy in faith, which is also major kufr and capable of sending one out of the fold of Islam. This hypocrisy is subdivided into six: Belying Allah’s Messenger, belying any part of his message, harboring hatred against Allah’s Messenger, harboring hatred against his message, being happy when the religion preached by Allah’s Messenger suffers a set-back, or being unhappy when his mission attains any victory.
The second type is practical hypocrisy. This is minor kufr and so it does not nullify the perpetrators faith, even though it is a capital offense. Some examples of practical hypocrisy are mentioned in the following ḥadīth,
“Whoever has the following four [characteristics] will be a pure hypocrite and whoever has one of the following four characteristics will have one characteristic of hypocrisy unless and until he gives it up: Whenever he is entrusted, he betrays; whenever he speaks, he tells a lie; whenever he makes a covenant, he proves treacherous; whenever he quarrels, he behaves in a very imprudent, evil and insulting manner.”
The Prophet also said,
“The signs of a hypocrite are three: when he speaks, he lies; when he promises, he always breaks it; if you trust him, he proves to be dishonest.”
As for minor kufr, then it refers to unbelief that neither strips the perpetrator of his faith nor does it subject him to eternal punishment in Hell, but constitutes a dire threat of being punished by Allah. This type of unbelief is also as kufr al-ni‘mah (disbelief of ingratitude). Any mention of the word kufr in the religious texts must be understood in this context if the related act falls short of what we can term as major kufr. Examples of this include what is mentioned in the following verse,
“Allah presents an example: a city which was safe and secure, its provision coming to it in abundance from every location, but it denied the favors of Allah. So, Allah made it taste the envelopment of hunger and fear for what they had been doing.” (Q, 16:112)
It has been related in some ḥadīths,
“Two [things] are found among people which are tantamount to unbelief: slandering one’s lineage and lamentation on the dead.”
“Do not return to unbelief after me by striking the necks of one another.”
The kufr mentioned in the above texts, and other similar ones, is minor kufr, whose perpetrator remains a Muslim. Allah also says,
“If two groups among the believers should fight, then make settlement between the two. But if one of them oppresses the other, then fight against the one that oppresses until it returns to the ordinance of Allah. And if it returns, then make settlement between them in justice and act justly. Indeed, Allah loves those who act justly.” (Q, 49:9)
Thus, Allah refers to both groups as believers in spite of them fighting against one another. He also says,
“Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allah has certainly fabricated a tremendous sin.” (Q, 4:48)
This verse indicates that, based on what Allah may will, the perpetrator of any sin, except for shirk, will either be completely pardoned by Allah, or he will be subjected first to punishment in proportion to his misdeed. As for shirk, the verse has likewise stated plainly that it is an unpardonable offense in the sight of Allah. Allah also says,
“Whoever ascribes any partner to Allah, Allah has prohibited for him Paradise, and his shelter is the Fire; there will be no supporters for the wrongdoers.” (Q, 5:72)
 Bukhārī no. 34 and Muslim no. 58.
 Bukhārī no. 33.
 Muslim no. 67.
 Bukhārī no. 121 and Muslim no. 65.