The “sin” in question here is shirk, or the ascribing of partners unto Allah. The first verse quoted below spells this out for Muslims, so as to leave no doubt about the “unforgivability” of that sin.
“Verily, Allah does not forgive the ascribing of partners unto him (shirk), but for (sins) other than that he (can) forgive those whom he will, while he who has ascribed partners unto Allah has indeed contrived a tremendous sin” (Qur’an 4:45).
Now, notice how, in the following four verses the Qur’an itself does exactly that:
“And when they appeal unto Allah and his messenger to judge between them (at judgement day), Lo! A fraction of them are averse:
But, if right had been with them, they would have come unto him (him here could mean either Allah or Muhammad) submissively.
Is there in their hearts a disease, or did they doubt, or do they fear that Allah and his messenger should wrong them (on judgement day)? Nay, those are the wrong doers.
Verily, that saying of those who believe when summoned before Allah and his messenger to judge between them is that they say “we heard, and we obeyed,” and those are the successful ones” (on the day of judgment) (Qur’an 24: 48-51).
Note that both 4:45, and the group of 24: 48-51, are “Medina” verses, meaning that the law of abrogation, whereby many “Mecca” verses are negated by “Medina” verses, can not be applied here. Muslims have no choice but to accept this glaring contradiction.
The 48th verse of the 4th sura (chapter) clearly states that the sin of ascribing partners unto Allah is the worst of all possible sins, the only sin that Allah can not forgive. Yet, the 48th through the 51st verses of chapter 24 in the same Qur’an has Muhammad (the messengers) sitting side-by-side with Allah to judge the souls as they pass before that divine duo. In fact, it does appear that Muhammad has actual veto power over Allah’s decisions as judgement is passed on the souls.
The verses preceding this section in the 24th sura make it clear that the topic being discussed is the “Last Judgment Day.” This is confirmed by the tafaseer (explanations) by, ibn al-Khateeb (page 433), which has been officially approved by Egypt’s al-Azhar University, the “Vatican” equivalent for Sunni Islam. Al-Khateeb adds the comment that the reason that the messenger) Muhammd) will play this role at Judgement day is because “the messenger tells nothing but the truth.” But that still begs the question as to why God would even need Muhammad’s help in judging the souls. Does Muhammad know more about what’s in peoples’ hearts than does Allah?
This interpretation (this this is “Judgement Day” at the end of time is also confirmed by the more modern and al-Azhar approved translation by Muhammad Asad (p. 606), and is accepted by virtually all Islamic scholars from the Middle Ages to the present day.
These verses in the 24th sura, then make Muhammad at least equal to Allah and also a partner unto Allah (shirk) the greatest of all sins according to 4:48!
Islam thus negates itself as a religion, because it is the most outrageous of ideologies in terms of ascribing partners unto Allah.