التفسير التبليغي
Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. And whoever associates others with Allah has certainly gone astray a far straying. (إِنَّ اللَّهَ لا يَغْفِرُ أَنْ يُشْرَكَ بِهِ وَيَغْفِرُ مَا دُونَ ذَلِكَ لِمَنْ يَشَاءُ وَمَنْ يُشْرِكْ بِاللَّهِ فَقَدْ ضَلَّ ضَلالاً بَعِيداً)
The most dangerous of sins is for a person to associate partners with Allah, for *shirk* (polytheism/associating partners with Allah) is the greatest of sins.
Faith requires strengthening and protection from *shirk*, for Allah Almighty said: (وَمَنْ يُشْرِكْ بِاللَّهِ فَقَدْ ضَلَّ ضَلالًا بَعِيدًا), meaning that whoever associates partners with Allah in His worship has indeed deviated from the path of truth and swerved far from the straight path of guidance, obeying Satan and following his way, abandoning obedience to Allah and the methodology of His religion.
Source: التفسير التبليغي