Surah 4. An-Nisaa', Ayah 150

Or: "We believe in some and we deny the others" - that is, they believe in God but not in His apostles (Zamakhshari) or, alternatively, they believe in some of the apostles and deny others (Tabari and Zamakhshari). To my mind, the first of these two interpretations is preferable inasmuch as it covers not only a rejection of some of the apostles but also a total rejection of the idea that God may have revealed His will through His chosen message-bearers. In Islam, the rejection of any or all of God's apostles constitutes almost as grave a sin as a denial of God Himself.

 By their claim that they believe in Allah but still reject some of His messengers.

Unbelief takes various forms. Three are mentioned here: (1) denial of Allah and His revelation to mankind through inspired men; (2) a sort of nominal belief in Allah and His Prophets, but one which is partial, and mixed up with racial pride, which does not allow of the recognition of any Messengers beyond those of a particular race; and (3) a nominal belief in universal revelation, but so hedged round with peculiar doctrines of exclusive salvation, that it practically approaches to a denial of Allah's universal love for all mankind and all Creation. All three amount to Unbelief, for they really deny Allah's universal love and care.