The most enormous sin is blasphemy (Kufur). Shirk is a type of blasphemy; it is to worship other than Allah.  Surat Luqman, Ayah 13 contains the meaning of what Luqman said is, “O my son! Do not worship other than Allah, for shirk is extreme injustice.” It was narrated that “The Prophet, peace be upon him, was asked, ‘What is the most enormous sin?’ He said, ‘To attribute partners to Allah, the One Who created you.’” (Related by al-Bukhariyy and others)