The word "kâfir" means "unbeliever". It is applied to one who knowingly denies monotheism, prophecy, the legitimacy of Prophet Muhammad (peace be upon him), or all three.
The term is sometimes applied in the Qur'ân for those who eschew the law of Islam and turn away from thanking Allah. Allah says: “Those who reject faith will suffer from that rejection.” [Sûrah al-Rûm: 44].
This word is also used in the Qur'ân to mean a denial of Allah’s favor. Allah says: “His endeavor will not be rejected” [Sûrah al-Anbiyâ’: 94] and says: “If you are grateful, I will add more (favours) unto you; but if you show ingratitude, truly My punishment is terrible indeed.” [Sûrah Ibrâhîm: 7]
There are two kinds of unbelief:
a. Extreme unbelief which takes a person out of the fold of Islam.
b. Lesser unbelief which does not take a person out of the fold of Islam.
Under this second category falls the rejection of Allah’s favors.
The term unbelief is used in the Qur'ân in the context of a person who makes a show of unbelief while not subscribe to it inhis heart. Allah says: “And one who, after accepting faith in Allah utters unbelief, except under compulsion, His heart remaining firm in faith.” [Sûrah al-Nahl: 106]
Also, we can say that a person disbelieves in Satan if he rejects Satan or refuses to obey Satan. Allah says: “Whoever rejects (disbelieves in) false worship and believes in Allah has grasped the most trustworthy handhold that never breaks.” [Sûrah al-Baqarah: 256]