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            The Quran identifies Muhammad as a sinner: 40:55, 48:2, and 47:19:
40:55   Therefore have patience; God’s promise is surely true. Implore forgiveness for your sins, and celebrate the praise of your Lord evening and morning.
48:2     ... so that God may forgive you your past and future sins, and perfect His goodness to you; ...
Below is the Quran 47:19 in five English translations, (NOTE: Muhammad is asking for forgiveness for both himself and his followers).
            "Know that there is no deity but God. Implore Him to forgive your sins and to forgive the true believers, men and women. God knows your busy haunts and resting places." Dawood [3]
            "So know (O Muhammad) that there is no God save Allah, and ask forgiveness for thy sin and for believing men and believing women. Allah knoweth (both) your place of turmoil and your place of rest." Pickthall [4]
            "Know thou therefore that there is no god but God, and ask forgiveness for thy sin, and for the believers, men and women. God knows your going to and fro, and your lodging." Arberry [5]
            "Know, then, that there is no god but God; and ask pardon for thy sin, and for believers, both men and women. God knows your busy movements and your final resting places." Rodwell [6]
            "Know therefore that there is no god but Allah and ask forgiveness for the fault and for the men and women who believe: for Allah knows how ye move about and how ye dwell in your homes." Ali [7]
            The Quranic word used in these verses for sin is "dhanb", or "thanb", depending on how one transliterates the word. It is used many times in the Quran. The Hughes Encyclopedic Dictionary of Islam defines the word as "a sin or a crime, or the charge of such", [8].
            Clearly the Quran identifies Muhammad as a sinner. It is definitely applied to Muhammad. Since some Muslims argue that Muhammad was not a sinner, they have to resort to re-interpreting the Quranic word, "dhanb". So, what does that word "dhanb" mean in the context of the Quran? We have to address the true meaning of this word — is it "sin", meaning something that God will judge, or is it "fault" defined as meaning something that is very minor and easily excusable by God? To understand the context of the word, we should let the context of how the word is used throughout the Quran speak.
We should let the Quran interpret itself.
            I have found the word used about 39 times in the Quran. In some cases it is used without a reference, e.g. 3:31, "Say, "If you love God, follow me. God will love you and forgive your sins…". We have no reference as to what those "sins" consisted.
            However, "dhanb" is used many other times with a contextual reference.  Here is a significant selection.  I present about 22 of them because I want to establish exactly the Quran’s severity of this word.  And, I encourage all Muslims to search through their Quran to find an example of Allah lightly treating "dhanb".  Study the Quran to find something contrary to the harsh judgment that Allah proscribes to those who commit of "dhanb". I have found none.

3:11     (Their plight will be) no better than that of the people of Pharaoh and their predecessors: they denied Our Signs and Allah called them to account for their sins. For Allah is strict in punishment.
            Allah called Pharaoh’s people to account for their sins, and they were punished for their "dhanb".
3:16     (Namely) those who say: "Our Lord! we have indeed believed: forgive us then our sins and save us from the agony of the fire."
            Here we see people asking forgiveness for their "dhanb" because without forgiveness of their "dhanb" they will end up in the agony of fire (hell).
5:18     (Both) the Jews and the Christians say: "We are sons of Allah and His beloved." Say: "Why then doth He punish you for your sins? Nay ye are but men,- of the men He hath created: He forgiveth whom He pleaseth and He punisheth whom He pleaseth: and to Allah belongeth the dominion of the heavens and the earth and all that is between: and unto Him is the final goal (of all)."
            Here Muhammad is saying that Christians and Jews are being punished by Allah for the "dhanb". Thus their "dhanb" earns punishment from Allah, not a wink of an eye.
5:49     And this (He commands): Judge thou between them by what Allah hath revealed and follow not their vain desires but beware of them lest they beguile thee from any of that (teaching) which Allah hath sent down to thee. And if they turn away be assured that for some of their crimes it is Allah's purpose to punish them. And truly most men are rebellious.
            Here "dhanb" is defined as "crimes". Does any sincere person think that a criminal act is more than just a "mistake"? Of course not. How do you think God views "crimes".
6:6       See they not how many of those before them We did destroy? Generations We had established on the earth in strength such as We have not given to you for whom We poured out rain from the skies in abundance and gave (fertile) streams flowing beneath their (feet): yet for their sins We destroyed them and raised in their wake fresh generations (to succeed them).
            God is destroying generations of people for their "dhanb".
7:100   To those who inherit the earth in succession to its (previous) possessors is it not a guiding (lesson) that if We so willed We could punish them (too) for their sins and seal up their hearts so that they could not hear?
            Here the implication is that people could be punished by God for their "dhanb". "Dhanb" is worthy of punishment.
8:52     "(Deeds) after the manner of the people of Pharaoh and of those before them: they rejected the signs of Allah and Allah punished them for their crimes: for Allah is strong and strict in punishment:
            Again, people were punished by God for their "dhanb = crimes". Allah is "strong and strict" in dishing out punishment for people’s "dhanb".
8:54     "(Deeds) after the manner of the people of Pharaoh and those before them": they treated as false the signs of their Lord; so We destroyed them for their crimes and We drowned the people of Pharaoh: for they were all oppressors and wrong-doers.
            Again, for their "dhanb = crimes" people were destroyed and drowned by God. These people who committed "dhanb" were "oppressors and wrong-doers".
9:102   Others (there are who) have acknowledged their wrong-doings: they have mixed an act that was good with another that was evil. Perhaps Allah will turn unto them (in mercy): for Allah is Oft-Forgiving Most Merciful.
            Ali’s note says: "These were some whose will was weak and succumbed to evil, although there was much good in them ..." Ali calls these "dhanb" evil. Is "evil" the same as a "forgetful mistake"?
12:29   "O Joseph pass this over! (O wife) ask forgiveness for thy sin for truly thou hast been at fault!"
            Here we have a women who tried to seduce Joseph, then falsely accused him of trying to sexually molest her, being instructed to ask forgiveness of her "dhanb". Is lying a simple "mistake"? No. It is a sin before man and God.
12:97 They said: "O our father! ask for us forgiveness for our sins for we were truly at fault."
            Here we have Joseph’s brothers asking Jacob to ask God to forgive their "dhanb". What was their "dhanb"? They kidnapped Joseph and sold him into slavery. Then they lied about it to their father. Was this a simple "mistake", or a "memory lapse"? No, it was a brutal act, motivated by jealousy.
14:10   Their apostles said: "Is there a doubt about Allah the Creator of the heavens and the earth? It is He Who invites you in order that He may forgive you your sins and give you respite for a term appointed!" They said: "Ah! ye are no more than human like ourselves! Ye wish to turn us away from the (gods) our fathers used to worship: then bring us some clear authority."
            Here "dhanb" refers to idolatry. Is idolatry a mere "forgetful" act? And forgiveness of "dhanb" will save them from hellfire. Thus their "dhanb" will send them to hell without forgiveness.
26:14   "And they have a charge of crime against me; and I fear they may slay me."
            Here is Moses speaking. He said that he was charged with a "dhanb = crime". What was the crime? Murder. Would any Muslim say that murder is something that God overlooks?
28:78   He said: "This has been given to me because of a certain knowledge which I have." Did he not know that Allah had destroyed before him (whole) generations which were superior to him in strength and greater in amount (of riches) they had collected? But the wicked are not called (immediately) to account for their sins.
            Here, the reference is to Allah destroying entire generations on account of their "dhanb". Does Allah destroy entire generations because of simple mistakes?
29:40   Each one of them We seized for his crime: of them against some We sent a violent tornado (with showers of stones); some were caught by a (mighty) Blast; some We caused the earth to swallow up; and some We drowned (in the waters): it was not Allah Who injured (or oppressed) them: they injured (and oppressed) their own souls.
            Allah says he seized people for their "dhanb = crime". Look at how he punished them: violent tornado, showers of stones, mighty blast, swallowed by the earth, drowned in the water. Aren’t these "dhanb" bad? Does "dhanb" sound like it is something that God ignores? No. Rather, God sternly punishes "dhanb".
40:11, 12         They will say:" Our Lord! twice hast Thou made us without life and twice hast Thou given us Life! Now have we recognized our sins: is there any way out (of this)?" (The answer will be:) "This is because when Allah was invoked as the only (object of worship) ye did reject Faith but when partners were joined to Him ye believed! The command is with Allah Most High Most Great!"
            Here people admit their "dhanb = sins". How did allah define their sins? Their "dhanb" was that they joined partners to Allah. This type of sin is called shirk. It is considered by many Muslims to be unforgivable.
40:21   Do they not travel through the earth and see what was the End of those before them? They were even superior to them in strength and in the traces (they have left) in the land: but Allah did call them to account for their sins and none had they to defend them against Allah.
            Allah is judging people for their "dhanb = sins". Look, their "dhanb" was severe enough that they needed defense against Allah, but they had none.
46:31   "O our people hearken to the one who invites (you) to Allah and believe in him: He will forgive you your faults and deliver you from a Penalty Grievous.
            Here, Allah may forgive "dhanb = faults" and deliver people from a grievous penalty. So, these "dhanb" warrant a grievous penalty in Allah’s eyes.
61:12   He will forgive you your sins and admit you to Gardens beneath which rivers flow and to beautiful mansions in Gardens of Eternity: that is indeed the supreme achievement.
            Here, Allah is going to forgive "dhanb = sins". Afterwards they will be admitted into paradise. The people’s "dhanb" is going to keep them out of paradise unless they are forgiven.
67:11   They will then confess their sins: but far will be (Forgiveness) from the Companions of the Blazing Fire!
            People will be confessing their "dhanb = sins" but they will receive no forgiveness for them. Instead, these people are in hell on account of their sins.
91:14   Then they rejected him (as a false prophet) and they hamstrung her. So their Lord on account of their crime obliterated their traces and made them equal (in destruction High and low)!
            Here people rejected a prophet as a false prophet and killed a special camel. What did Allah do on account of their "dhanb = crime"? He "obliterated their traces", i.e., Allah totally destroyed them! This was on account of their "dhanb".
Let’s review. "Dhanb" is translated as sin, crime, & fault. How does Allah deal with people who have committed dhanb? Allah -

Obliterates them
Destroyed them
Puts them in hellfire
Gives them a grievous penalty
Calls them to account
Hits them with a violent tornado
Hits them with a mighty blast
Has the earth swallow them up
Drowns them in water
Punishes them
Puts them in the agony of fire
Allah does all this to people who commit "dhanb". Does this sound like something Allah passes over as a minor "stumbling", "mistake", or "act of forgetfulness"?
            Of course not! It sounds like Allah is severely punishing people for their "dhanb". It sounds like Allah is really serious about it. Don’t you agree?
            And remember, Muhammad committed "dhanb". Muhammad was told to ask forgiveness for his "dhanb". And Muhammad taught in the Quran that forgiveness of "dhanb" was required to get into Paradise. Otherwise, a person with unforgiven "dhanb" ends up in hell.