شرك
Shirk
Associating partners with God: giving to anything in creation what belongs to God alone.
What it means
Shirk means “partnership”. It is the one sin the Quran says God does not forgive without repentance (4:48), and Luqman tells his son that it is “a great injustice” (31:13). The Prophet listed it first among the gravest sins (Sahih al-Bukhari).
The word covers worshipping idols, but also treating wealth, status or one’s own desires as a rival to God in the heart.
Often misunderstood as
That every non-Muslim is condemned as a pagan.
What the sources say
The Quran itself separates the categories. It speaks of “those who disbelieved among the People of the Book and the polytheists” as two groups (98:1), and it calls Jews and Christians by their own name, People of the Book. Classical scholars discussed at length whether and how Trinitarian belief falls under shirk; there was no single ruling that lumped everyone together.
Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 37, listed as SHIRK; p. 37, listed as MUSHRIK. The definition is restated here.