Sectarianism = Shirk

TLDR: The Quran says those who divide their religion into sects are committing idol worship. Sunni Islam is a sect; therefore, it is a form of idol worship.

[30:31] You shall submit to Him, reverence Him, observe the Contact Prayers (Salat), and—whatever you do—do not ever fall into idol worship.

[30:32] (Do not fall in idol worship,) like those who divide their religion into sects; each party rejoicing with what they have.

(٣١) مُنِيبِينَ إِلَيْهِ وَٱتَّقُوهُ وَأَقِيمُوا۟ ٱلصَّلَوٰةَ وَلَا تَكُونُوا۟ مِنَ ٱلْمُشْرِكِينَ

(٣٢) مِنَ ٱلَّذِينَ فَرَّقُوا۟ دِينَهُمْ وَكَانُوا۟ شِيَعًا كُلُّ حِزْبٍۭ بِمَا لَدَيْهِمْ فَرِحُونَ