pl. kafirun. Lit. “The coverer.” One who hides or covers up the truth.
The word is generally used by Muslims to define one who is an unbeliever in the ministry of Muhammad and his Qur’an, and in this sense it seems to have been used by Muhammad himself. Surah ii. 37: “Those who misbelieve (wa’llazina kafaru), and call our signs lies, they are fellows of the Fire, they shall dwell within for ever.”
It is also used for those who believe in the Divinity of the Lord Jesus, and the Holy Trinity, Surah v. 76: “They indeed are infidels (la-gad kafara ‘llazina), who say God is al-Masibu ibn Maryam…. Verify him who associates anything with God, hath God forbidden Paradise, and his resort is the Fire.”
Sura v. 77: “They are infidels who say Verily God is the third of three.”
[On this passage the Kamalan say it refers to the Nestorians and to the Malaka'iyah, who believe that God is one of three, the other two being the mother and son.]
According to the Raddu ‘l-Muhtar (vol. iii. p. 442), there are five classes of kafirs or infidels: (1) Those who do not believe in the Great First Cause; (2) Those who do not believe in the Unity of God, as the Sanawiyah who believe in the two eternal principles of light and darkness; (3) Those who believe in the Unity of God, but do not believe in a revelation; (4) Those who are idolaters; (5) Those who believe in God and in a revelation, but do not believe in the general mission of Muhammad to the whole of mankind, as the Christians, a sect of the Jews (sic).
Saiyid Sharif Jurjani says : “Mankind are divided into two parties, namely, those who acknowledge the mission of Muhammad, and those who do not believe in it. Those who do not believe in his mission are either those who reject it and yet believe in the inspiration and divine mission of other prophets, as the Jews and Christians, and also the Majusi (Fire Worshippers); or those who do not believe in any revelation of God’s will. Those who do not believe in any revelation from God, are either those who acknowledge the existence of God, as the Brahma (Buddhists?), or those who deny the existence of a Supreme Ruler, as the Dahri or Atheists.”
“Those who do not acknowledge Muhammad as an inspired prophet are either those who do it wilfully and from mere enmity, or those who do not acknowledge it from reflection and due to study of the subject. For the former is eternal punishment, and for the latter that punishment which is not eternal. There are also those who, whilst they are Muslims, are not orthodox in their belief; these are heretics, but they are not kafirs. Those who are orthodox are an-Naji or the salvationists.” (Sharhu ‘l-Muwaqif, p. 597.)
Based on Hughes, Dictionary of Islam