(The Unforgivable Sin)
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.
إِنَّ اللَّهَ لا يَغفِرُ أَن يُشرَكَ بِهِ وَيَغفِرُ ما دونَ ذٰلِكَ لِمَن يَشاءُ وَمَن يُشرِك بِاللَّهِ فَقَد ضَلَّ ضَلٰلًا بَعيدًا
innal laaha laa yaghfiru ai yushraka bihee wayaghfiru maa doona zaalika limai yashaaa’; wa mai yushrik billaahi faqad dalla dalaalam ba’eedaa
*4:116 A simple definition of idolatry: Believing that anything beside God can help you.