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"Pickthall: 52So obey not the disbelievers, but strive against them herewith with a great endeavour."
"[T]he term kaafir cannot be simply equated, as many Muslim theologians of post-classical times and practically all Western translators of the Qur’aan have done, with “unbeliever” or “infidel” in the specific, restricted sense of one who rejects the system of doctrine and law promulgated in the Qur’aan and amplified by the teachings of the Prophet — but must have a wider, more general meaning This meaning is easily grasped when we bear in mind that the root verb of the participial noun kaafir (and of the infinitive noun kufr) is kafara, “he [or “it”] covered [a thing]”: thus, in 57:20 the tiller of the soil is called (without any pejorative implication) kaafir, “one who covers,” ie, the sown seed with earth, just as the night is spoken of as having “covered” (kafara) the earth with darkness In their abstract sense, both the verb and the nouns derived from it have a connotation of “concealing” something that exists or “denying” something that is true Hence, in the usage of the Qur’aan — with the exception of the one instance (in 57:20) where this participial noun signifies a “tiller of the soil” - a kaafir is “one who denies [or “refuses to acknowledge”] the truth” in the widest, spiritual sense of this latter term: that is, irrespective of whether it relates to a cognition of the supreme truth — namely, the existence of God - or to a doctrine or ordinance enunciated in the divine writ, or to a self-evident moral proposition, or to an acknowledgment of, and therefore gratitude for, favours received"

Kaafir is an Islamic term of Arabic origin used by Muslims to refer to non-Muslims who deny the God in Islam, reject his authority, and do not accept the message of Islam as truth
"Pickthall: 52So obey not the disbelievers, but strive against them herewith with a great endeavour."
"Ali: 29Fight those who believe not in Allah nor the Last Day, nor hold that forbidden which hath been forbidden by Allah and His Messenger, nor acknowledge the religion of Truth, (even if they are) of the People of the Book, until they pay the Jizya with willing submission, and feel themselves subdued."
"Ali: 28O ye who believe! The idolaters only are unclean. So let them not come near the Inviolable Place of Worship after this their year. If ye fear poverty (from the loss of their merchandise) Allah shall preserve you of His bounty if He will. Lo! Allah is Knower, Wise."
"Remember thy Lord inspired the angels (with the message): "I am with you: give firmness to the Believers: I will instil terror into the hearts of the Unbelievers: smite ye above their necks and smite all their finger-tips off them." This because they contended against Allah and His Messenger: If any contend against Allah and His Messenger, Allah is strict in punishment. Thus (will it be said): "Taste ye then of the (punishment): for those who resist Allah, is the penalty of the Fire."
"Ali: 17It is not for such as join gods with Allah, to visit or maintain the mosques of Allah while they witness against their own souls to infidelity. The works of such bear no fruit: In Fire shall they dwell."
"For the worst of beasts in the sight of Allah are those who reject Him: They will not believe."