Inna allatheena kafaroo wamatoo wahum kuffarun ola-ika AAalayhim laAAnatu Allahi walmala-ikati wannasi (surah Baqarah: 161)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(2:161) Those who adopted the way of disbelief *161 and died as disbelievers, are accursed of Allah and of angels and of all mankind: they shall remain accursed for ever.

Innal lazeena kafaroo wamaa too wa hum kuffaarun ulaaa`ika `alaihim la `natul laahi walmalaa`ikati wannaasi ajma`een

*161). The original meaning of kufr is to conceal. This lent the word a nuance of denial and it began to be used as an antonym of iman . Iman means to believe, to accept, and to recognize. Kufr, on the contrary, denotes refusal to believe, to deny, to reject. According to the Qur'an there are several possible forms of disbelief. One is to refuse either to believe in the existence of God, to acknowledge His sovereignty, to recognize Him as the only Lord of the Universe and of mankind or to recognize Him as the only Lord and the only object of worship and adoration. The second form of disbelief is when a man recognizes the existence of God but still refuses to accept His ordinances and directives as the only source of true guidance, and as the true law for his life. The third form of disbelief is when even though a man recognizes in principle that he ought to follow the guidance of God he refuses to believe in the Prophets who were the means of communicating God's guidance to man. The fourth form of disbelief is to differentiate between one Prophet and another and, out of parochialism and bigotry, to accept some Prophets and reject others. The fifth form of disbelief is the refusal to recognize, either totally or partially, the teachings communicated by the Prophets, on God's behalf, concerning the beliefs, the principles of morality, and the laws for fashioning human life. The sixth form of disbelief is where a person theoretically accepts all that he should accept but wilfully disobeys God's ordinances and persists in this disobedience, and considers disobedience rather than obedience to God to be the true principle of life.
All these modes of thought and action are forms of rebellion towards God and the Qur'an characterizes each of them as kufr. In addition, the term kufr is used at several places in the Qur'an in the sense of ingratitude and in this usage it signifies the exact opposite of shukr (gratitude). Gratefulness consists in feeling thankful to one's benefactor, in duly recognizing the value of his benevolence, in making use of it in a manner pleasing to its bestower, and in being utterly loyal to him. Ingratitude denotes, on the contrary, that the man concerned has either failed to recognize the bounty of his benefactor, or considers it to be either the outcome of his own power and ability or else that of some other person's favour or intercession. Another form of ingratitude consists in failing to recognize fully the worth of the bounty received and in either allowing it to be wasted or to be used in contravention of the will of the benefactor; this ingratitude includes infidelity and treachery towards the benefactor in disregard of his benevolence and kindness. This kind of kufr is known in ordinary language as ungratefulness.