The One Unforgiveable Sin is Shirk

Associating other deities with Allah is the one unforgiveable sin in Islam.

Allah T'ala says in the Holy Quran:

إِنَّ اللّهَ لاَ يَغْفِرُ أَن يُشْرَكَ بِهِ وَيَغْفِرُ مَا دُونَ ذَلِكَ لِمَن يَشَاء وَمَن يُشْرِكْ بِاللّهِ فَقَدِ افْتَرَى إِثْمًا عَظِيمًا

004.048 God forgives not that partners should be set up with Him; but He forgives
anything else, to whom He pleases; to set up partners with God is to devise a sin
Most heinous indeed.

The purpose of this verse is not to tell man that he may commit any sin as long as he does not associate others with God in His divinity. The object is rather to impress upon those who had begun to regard polytheism as a trivial matter that it constitutes the most serious offence in God's sight, an offence so serious that while other sins may be pardoned this will not.

The Arabic word Shirk can be translated as polytheism. Polytheism should be understood in the light of the Islamic Holy texts.

From: Muhammad