Surah 4. An-Nisaa', Ayah 48

Lit., "anything below that".

The continuous stress, in the Qur'an, on God's transcendental oneness and uniqueness aims at freeing man from all sense of dependence on other influences and powers, and thus at elevating him spiritually and bringing about the "purification" alluded to in the next verse. Since this objective is vitiated by the sin of shirk ("the ascribing of divine qualities to aught beside God") the Qur'an describes it as "unforgivable" so long as it is persisted in, i.e., unless and until the sinner repents (cf. verses {17} and {18} of this surah).

 A person will never be forgiven if they die as a disbeliever. But if they repent before their death and mend their ways, then their repentance will be accepted (see 25:68-70).

Just as in an earthly kingdom the worst crime is that of treason, as it cuts at the very existence of the State, so in the Divine Kingdom, the unforgivable sin is that of contumacious treason against Allah by putting up Allah's creatures in rivalry against Him. This is rebellion against the Creator. It is what Plato would call the "lie in the soul." But even here, if the rebellion is through ignorance, and is followed by sincere repentance and amendment, Allah's Mercy is always open (iv. 17).