In the very beginning, we would like to make it clear that the word kufr has been subject to many misunderstandings. Islam makes it a horrendous sin to label someone kafir without having any legal proof to say so. Thus, we should make difference between various kinds of kufr and understand each kind according to its true concept.

To make the concept of kufr clear, we would like to cite for you what the renowned Muslim scholar, Sheikh Yusuf Al-Qaradawi, states in his book ‘Islamic Awakening Between Rejection and Extremism’ :Many misconceptions that control the minds of the Muslim youth need to be addressed and fully explained so as to make clear the rulings of Islam and to avoid any form of baseless dangerous judgments. Some of the most important misconceptions include concepts like the concept of Islam, iman, kufr (disbelief), nifaq (hypocrisy) and jahiliyyah (paganism)…etc. Linguistic complexities or a lack of mastery of the Arabic language by some people have led to such confusion and misunderstanding.

The complexities of language escape the non-experts. Consequently, they become unable to differentiate between the figurative or metaphoric meanings and the literal meanings, thereby confusing matters. They are unable to realize, for instance, the difference between absolute (or perfect) iman and limited (or nominal) iman; between perfect Islam and limited (or nominal) Islam between major kufr leading the person totally outside the fold of Islam and the kufr of disobedience between major shirk and minor shirk; between hypocrisy of belief and hypocrisy of action. They also equate the jahiliyyah of ethics and behavior with that of belief. The following is a brief clarification of these concepts with a view to preventing dangerous consequences.
In juristic language, kufr signifies the rejection and denial of Allah, Glory be to Him, and His messages, according to the following Qur’anic verse: “Anyone who denies Allah, His angels, His books, His Apostles, and the Day of judgment has gone far, far astray.” (An-Nisaa’: 136)

Kufr could also imply apostasy, and consequently the complete loss of iman.

The term kufr is also used to refer to transgression which is far short of a total rejection of Islam and do not amount to a rejection and denial of Allah, Glory be to Him, and His Messenger. The well known Muslim scholar Imam Ibn Al-Qayyim divided kufr into two categories namely major kufr and minor kufr. Major kufr leads to eternal punishment in the Hellfire while minor kufr leads to temporary rather than eternal punishment in the Hellfire. Consider the following Hadiths: “Two things, if practiced by my Ummah, are manifestations of kufr: false accusation about a person’s lineage and lamentation of the deed.” And: “He who has anal sex with his wife commits kufr in what was revealed to Muhammad.” Also: “If a person seeks a soothsayer or fortuneteller and believes in him or her, he commits kufr in what was revealed to Muhammad.” In another Hadith, we read: “Do not revert to kufr after my death by putting one another to the sword.”
There is, in fact, a Qur’anic verse that is clearly misconceived by people. The verse reads: “Whoso judgeth not by that which Allah hath revealed: such are disbelievers.” (Al-Ma’idah: 44)

This verse is given various interpretations. Ibn `Abbas (may Allah be pleased with him) says: “It is not kufr which excludes a person from the fold of Islam, but it resembles kufr in one aspect, because the person who commits it does not deny Allah and the Last Day.” Tawus expressed the same opinion. `Atta’ said: “The kufr meant in the verse, refers to a form of kufr which can be greater or lesser in degree than another.”

Others, like `Ikrimah, argued that those who do not judge in accordance with what Almighty Allah has revealed commit kufr. But this argument is weak, because sheer rejection of Allah’s revelation amounts to kufr whether a person has judged in the light of the Shari`ah or not. `Abdul-`Aziz Al Kinani maintains that kufr comprises negligence to judge in accordance with all that which Allah, the Almighty, has revealed, including tawhid (monotheism) and Islam.

Ibn Al-Qayyim says: “Judging contrary to what Allah, the Almighty, has revealed involves both types of kufr, the major and the minor, according to the attitude of the person making the judgment. If the person believes that a judgment must be passed according to what Allah has revealed and a punishment decided, but refrains from doing so out of disobedience and transgression, in that case he commits minor kufr. But if he believes that it is not obligatory to judge according to the Shari`ah and that he is free to act, notwithstanding his conviction that it is divine, he then commits a major kufr. But if he acts out of ignorance, or makes an unintentional mistake, he is only to be judged as a wrongdoer.”
The gist of the matter is that all forms of transgression and disobedience are types of kufr implying ingratitude, because gratitude requires compliance and obedience. Therefore, human endeavor could be manifest in gratitude, or in kufr, or in something other than the two. Only Allah knows what is in the heart of everyone.