I think britexpat is right. One unforgivable sin. Same for Christianity. And as for some of the above comments - yes, many followers of Islam, Christianity, etc. are merely blind (figuratively speaking) followers and thus, not in reality true Muslims, Christians, etc. Anyway, what I found:-

http://wiki.answers.com/Q/Are_there_unforgivable_sins_in_Islam...

Yes, there is one unforgivable sin.... associating partners with Allah swt (God).

[Quran 4:48] GOD does not forgive idolatry, but He forgives lesser offenses for whomever He wills. Anyone who sets up idols beside GOD, has forged a horrendous offense.

[Quran 4:116] GOD does not forgive idol worship (if maintained until death), and He forgives lesser offenses for whomever He wills. Anyone who idolizes any idol beside GOD has strayed far astray.

[Quran 39:65] It has been revealed to you, and to those before you that if you ever commit idol worship, all your works will be nullified, and you will be with the losers.

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