"Verily We have warned you of a Penalty near the Day when man will see (the deeds) which his hands have sent forth and the unbeliever will say Woe unto me! Would that I were (mere) dust!²
Disbelief is the rejection of any of the pillars of Islamic Faith and it has two aspects:
(Qur'an: Translation of the meaning, 78:40)
Major disbelief (AlKufr al-Akbar): This aspect excludes one completely from the fold of Islam.
There are five types (of major disbelief):
1. Kufr at-Takdhib which is disbelieving the Divine truth or denying of any of the articles of Faith.
2. Kufr al-Iba wat-Takabbur ma'at-Tasdiq. This is rejecting and being too arrogant to submit to Allah's Commandments after being convinced of their truth.
"Then who does more wrong than one who utters a lie against Allah and denies the truth [this Qur'an, the Prophet (Muhammad), Islamic monotheism], when it comes to him. Is there not in Hell an abode for the disbelievers?²
(Qur'an: Translation of the meaning, 39:32)
3. Kufr ash-Shakk waz-Zann. This is doubt or lack of conviction in the six articles of Faith.
"And (remember) when We said to the angels: 'Prostrate yourself before Adam.' And they prostrated, except Iblis; he refused and was proud and was one of the disbelievers (disobedient to Allah)."
(Qur'an: Translation of the meaning, 2:34 ).
4. Kufr al-rad. This is turning away from the truth knowingly, or deviating from the obvious signs which Allah has revealed.
"And He went into his garden while in a state (of pride and disbelief) unjust of himself. He said: I think not that this will ever perish. And I think not the Hour will ever come, and if indeed I am brought back to my Lord (on the Day of Resurrection) I surely shall find better than this when you disbelieve in Him Who created you out of dust (i.e. your father Adam), then out of nutfah (mixed semen drops of male and female discharge), then fashioned you into a man?¹ But as for my part (I believe) that He is Allah, my Lord, and none shall I associate as partner with my Lord."
(Qur'an: Translation of the meaning, 18:35-38)
5. Kufr an-Nifaq. This is hypocritical disbelief.
"We created not the heavens and the earth and all that is between them except with truth and appointed term. But those who disbelive turn away from that whereof they are warned."
(Qur'an: Translation of the meaning, 46:3)
Minor disbelief (Al-Kufr-al-Asghar): This aspect of disbelief does not exclude one from the fold of Islam. It is also termed Kufrn-Ni'amah. This entails disbelief manifesting itself in ungratefulness for Allah's blessings or favors.
"They have made their oaths a screen (for their hypcrisy). Thus they hinder (men) from the path of Allah. Verily, evil is what they used to do. That is because they believed, then disbelieved, therefore their hearts are sealed, so they understand not."
(Qur'an: Translation of the meaning, 63:2-3)
"And Allah puts forward the example of a township (Makkah), that dwelt secure and well-content; its provision coming to it in abundance from every place, but it (its people) denied the favor of Allah (with ungratefulness). So Allah made it taste extreme of hunger (famine) and fear, because of the (evil, i.e. denying Prophet Muhammad) which they (its people) used to do."
(Qur'an: Translation of the meaning, 16:112)