There is no denying that the Quran asserts that Muhammad was an apostle and prophet:
Muhammad is not the father of any of your men, but (he is) the Apostle of God, and the Seal of the Prophets: and God has full knowledge of all things. S. 33:40 Y. Ali
What may not be apparent to our readers is that the Quran contradicts itself at this point since it claims that Muhammad was only a warner whom Allah sent:
Say: For myself I have no power to benefit, nor power to hurt, save that which Allah willeth. Had I knowledge of the Unseen, I should have abundance of wealth, and adversity would not touch me. I am but (illa) a warner, and a bearer of good tidings unto folk who believe. S. 7:188 Pickthall
So perchance you (Muhammad SAW) may give up a part of what is revealed unto you, and that your breast feels straitened for it because they say, "Why has not a treasure been sent down unto him, or an angel has come with him?" But you are ONLY a warner (innama anta natheerun). And Allah is a Wakil (Disposer of affairs, Trustee, Guardian, etc.) over all things. S. 11:12 Hilali-Khan
Those who disbelieve say: If only some portent were sent down upon him from his Lord! Thou art a warner ONLY (innama anta munthirun), and for every folk a guide. S. 13:7 Pickthall
And they say: "Why are not signs sent down to him from his Lord? Say: "The signs are only with Allah (qul innama al-ayatu AAinda Allahi), and I am ONLY a plain warner (wa-innama ana natheerun mubeenun)." S. 29:50 Hilali-Khan
Interestingly, the last three passages emphatically deny that Muhammad brought any portents since he was only a warner, implying that warners weren’t equipped with signs and wonders. For more on this issue please read this article on Muhammad and Miracles.
Say (O Muhammad SAW): "O mankind! I am (sent) to you ONLY as a plain warner (innama ana lakum natheerun mubeenun)." S. 22:49 Hilali-Khan
Say (unto them, O Muhammad): I am ONLY a warner (Qul innama ana munthirun), and there is no God save Allah, the One, the Absolute, S. 38:65 Pickthall
Say: "As to the knowledge of the time, it is with God alone (Qul innama alAAilmu AAinda Allahi): I am (sent) ONLY to warn plainly in public (wa-innama ana natheerun mubeenun)." S. 67:26 Y. Ali; cf. 17:105; 25:56; 27:92; 34:28; 79:45
The Arabic word innama is a particle that negates everything else, which in reference to Muhammad means that he is a warner and nothing else. The Muslim should have no problem accepting this point, that in these specific references innama means Muhammad is ONLY a warner and nothing more, since the Quran says something similar about the unity of Allah:
This is a clear message for mankind in order that they may be warned thereby, and that they may know that He is ONLY One God (annama huwa ilahun wahidun), and that men of understanding may take heed. S. 14:52 Pickthall
Allah hath said: Choose not two gods. There is ONLY One God (innama huwa ilahun wahidun). So of Me, Me only, be in awe. S. 16:51 Pickthall
And:
God bears witness that there is no god but (illa) He -- and the angels, and men possessed of knowledge -- upholding justice; there is no god but He, the All-mighty, the All-wise. S. 3:18
And We sent never a Messenger before thee except that We revealed to him, saying, 'There is no god but (illa) I; so serve Me.' S. 21:25 Pickthall
Which Muslim would take the above texts to mean that Allah is not the only god? Which Muslim would deny that only (innama) or but (illa) mean exactly that, i.e. that there is no other deity except Allah and that he alone is God? Yet the Quran uses the same expressions in reference to Muhammad being a warner. If the Muslim is to remain consistent s/he must accept the fact that Muhammad was only a warner, no more, no less.
Here is another example:
Say: I am ONLY a mortal like you (Qul innama ana basharun mithlukum). My Lord inspireth in me that your God is ONLY One God (annama ilahukum ilahun wahidun). And whoever hopeth for the meeting with his Lord, let him do righteous work, and make none sharer of the worship due unto his Lord. S. 18:110
Again, which Muslim would have a problem admitting that this text clearly says that Muhammad is only a human being, that he was not an angel, jinn or a divinity walking this earth?
A Muslim may respond to this by saying that Muhammad’s role as a warner automatically included his functions as prophet and messenger. After all, the Quran itself testifies that prophets and messengers were also warners:
Mankind were one community, and Allah sent (unto them) prophets as bearers of good tidings and as warners, and revealed therewith the Scripture with the truth that it might judge between mankind concerning that wherein they differed. And only those unto whom (the Scripture) was given differed concerning it, after clear proofs had come unto them, through hatred one of another. And Allah by His Will guided those who believe unto the truth of that concerning which they differed. Allah guideth whom He will unto a straight path. S. 2:213 Pickthall
Apostles who gave good news as well as warning, that mankind, after (the coming) of the apostles, should have no plea against God: For God is Exalted in Power, Wise. S. 4:165 Y. Ali
We send the apostles only to give good news and to warn: so those who believe and mend (their lives), - upon them shall be no fear, nor shall they grieve. S. 6:48 Y. Ali; cf. 18:56
The Muslim may therefore reason that in order for Muhammad to have been a warner he must have also been a prophet and messenger.
There are a couple of problems with such an ad hoc explanation. First of all, the classical Muslim commentators have discussed in considerable detail the differences in rank and function regarding the titles of prophet and messenger (cf. this article) but warner was never considered a special title or role, and certainly never put above the function of a prophet or messenger.
The second problem with this logic is that the Quran does not agree that every warner was a prophet or messenger. Note, for instance, what it says here:
And (remember) when We sent towards you (Muhammad SAW) Nafran (three to ten persons) of the jinns, (quietly) listening to the Qur'an, when they stood in the presence thereof, they said: "Listen in silence!" And when it was finished, they returned to their people, as warners. S. 46:29 Hilali-Khan
According to this above text some jinns went to their own people to warn them after hearing the recitation of the Quran. Specific passages of the Quran imply that jinn cannot be messengers or prophets due to the fact that Allah assigned these functions only to human beings, specifically to men:
We sent not forth any before thee, but men We revealed to of the people living in the cities. Have they not journeyed in the land? Have they not beheld how was the end of those before them?. Surely the abode of the world to come is better for those that are godfearing. What, do you not understand? S. 12:109 Arberry
And We sent none before thee, but men to whom We made revelation -- question the People of the Remembrance, if you do not know -- nor did We fashion them as bodies that ate not food, neither were they immortal; S. 21:7-8 Arberry; cf. 25:20-21
Moreover, the Quran speaks of Allah sending warners to Pharaoh’s people:
To the People of Pharaoh, too, aforetime, came Warners (from God). The (people) rejected all Our Signs; but We seized them with such Penalty (as comes) from One Exalted in Power, able to carry out His Will. S. 54:41-42 Y. Ali
The Quran mentions at least one particular warner who wasn’t a prophet or messenger, but an ordinary Egyptian who had come to faith:
Then said a certain man, a believer of Pharaoh's folk that kept hidden his belief, 'What, will you slay a man because he says, "My Lord is God," yet he has brought you the clear signs from your Lord? If he is a liar, his lying is upon his own head; but if he is truthful, somewhat of that he promises you will smite you. Surely God guides not him who is prodigal and a liar. O my people, today the kingdom is yours, who are masters in the land. But who will help us, against the might of God, if it comes upon us?' Said Pharaoh, 'I only let you see what I see; I only guide you in the way of rectitude.' Then said he who believed, 'My people, truly I fear for you the like of the day of the parties, the like of the case of Noah's people, Ad, Thamood, and those after them; and God desires not wrong for His servants. O my people, I fear for you the Day of Invocation, the day you turn about, retreating, having none to defend you from God; and whomsoever God leads astray, no guide has he. Joseph brought you the clear signs before, yet you continued in doubt concerning that he brought you until, when he perished, you said, "God will never send forth a Messenger after him." Even, so God leads astray the prodigal and the doubter.' … Then said he who believed, 'My people, follow me, and I will guide you in the way of rectitude. O my people, surely this present life is but a passing enjoyment; surely the world to come is the abode of stability. Whosoever does an evil deed shall be recompensed only with the like of it, but whosoever does a righteous deed, be it male or female, believing -- those shall enter Paradise, therein provided without reckoning. O my people, how is it with me, that I call you to salvation, and you call me to the Fire? You call me to disbelieve in God, and to associate with Him that whereof I have no knowledge, while I call you to the All-mighty, the All-forgiving. No doubt that what you call me to has no call heard, in this world or in the world to come, that to God we return, and that the prodigal are the inhabitants of the Fire. You will remember what I say to you. I commit my affair to God; surely God sees His servants.' So God guarded him against the evil things of their devising, and there encompassed the folk of Pharaoh the evil chastisement, the Fire, to which they shall be exposed morning and evening; and on the day when the Hour is come: 'Admit the folk of Pharaoh into the most terrible chastisement!' S. 40:28-34, 38-46 Arberry
The foregoing indicates that, as far as the Quran is concerned, even though all prophets and messengers were warners sent by Allah, not every warner was a prophet or messenger. In other words, a messenger and prophet is a warner but a warner isn’t necessarily a messenger or prophet.
Hence, for the Quran to say that Muhammad was ONLY a warner is to basically deny that he was also a prophet and messenger.
Now if the Quran had said that Muhammad was only a messenger or that he was only a prophet then there would be no problem since messengers and prophets were also warners to their people. But this is not what the Muslim scripture says of Muhammad.
As it stands, the Quran contradicts itself regarding Muhammad’s role and function, i.e. was he sent ONLY as a warner, or was he also a prophet and messenger? Both views can’t be correct (in fact, neither position is true since Muhammad was not sent by the true God, neither as warner nor as prophet or messenger).
Further Reading
http://answering-islam.org/Authors/Arlandson/roles_muhammad.htm
http://answering-islam.org/Quran/Contra/mo_not_universal.html
http://answering-islam.org/Quran/Contra/jonah.html
http://answering-islam.org/Quran/Contra/jinn_messengers.html
http://answering-islam.org/Quran/Contra/eat_or_not.html
Contradictions in the Qur'an
Answering Islam Home Page