Allah forgiveth not (The sin of) joining other gods with Him; but He forgiveth whom He pleaseth other sins than this: one who joins other gods with Allah, Hath strayed far, far away (from the right). An-Nisa 4:116 ⧉
Tafsir:
Opposition to the call of truth and victimization of the followers of the giver of that call are unforgivable offences in the eyes of God. While all other sinful acts may have been committed out of human weakness, the act of opposing the call of Truth is entirely due to insolence and haughtiness: insolence and rebellion are crimes that God will not forgive unless man confesses his mistakes and gives up his rebellious stance. Whenever the call of truth is brought to people in its pure and original form (without making concessions to people’s whims and wishes) it always is a divine mission supported by the special succour of God. To oppose such a mission is to stand in opposition to God Almighty. And can any individual succeed if he takes a stand against God? One who firmly believes in God has his actions rooted in Him. He may err occasionally but, when he turns to God, he rejoins his real roots. On the other hand, one who has associated himself with anyone besides God, is as if deprived of the true base in this universe. An individual who has not established his roots in the one God may perform some good actions, but such actions do not emanate from the divine source; they have only a surface value and are easily nullified by the slightest jolt. That is why a good action based in monotheism unfolds its results in the Hereafter, whereas that based in polytheism is swept away in this world itself.