There is only one one sin in which the sinner never does taubah (repentance), bid'ah. Because the sinner commits this grave sin thinking it is Islam, he thinks its an act of worship in order to get closer to Allah.

Narrated Abu Sa`id: The Prophet said, "You will follow the wrong ways, of your predecessors so completely and literally that if they should go into the hole of a mastigure (a lizard like reptile), you too will go there." We said, "O Allah's Apostle! Do you mean the Jews and the Christians?" He replied, "Whom else?" (Meaning, of course, the Jews and the Christians.)
[Sahih Bukhari Vol. 4, Bk. 56: Virtues and Merits of the Prophet (pbuh) and his Companions, Hadith#662]

People Are Addicted To Follow Their Ancestors As Jews n Christians Was.
A lecture by Shaikh Abdur Raheem Green