Surah 4

Surah 4:48

"Surely God does not forgive (anything) being associated with Him, but He forgives what is other than that for whomever He pleases. Whoever associates (anything) with God has forged a great sin."

4.48 – Surely God does not forgive (anything) being associated with Him. . . . Whoever associates (anything) with God has forged a great sin
Here “associating” (shirk) is singled out as the one unforgivable sin. This has implications for the way that Muslims view Christians, if Muslims believe that Christians “associate” Jesus – whom Muslims consider a mere human prophet – with God. See the summary of verses that accuse the People of the Book of “associating” at 61.9.

- from The Quran with Christian Commentary: A Guide to Understanding the Scripture of Islam