This is an idea that modern orientalists put forward; it is interesting, however that no one raised such objections at the time when Prophet Muhammad  was alive. Unlike the orientalists, no one at that time suggested that when the Prophet met with the monk Bahira during his childhood he had received certain religious information from him, and no one alleged that he had received religious information from Christian monks or Jewish rabbis during journeys he made with caravans.

Moreover, if he had received all his knowledge from the monk Bahira and during the journeys he made later when he was 25, then it could justifiably be asked why did Prophet Muhammad  not claim Prophet-hood until he was forty years old? The people of Mecca did not raise any such baseless allegations regarding the era prior to the Prophet-hood; rather their objections were only concerned with the era of the Prophet-hood.

The people in Mecca who made objections put forward arguments like: "This fellow does not know how to read or write, and thus he could not have obtained any knowledge from books. He has been living among us for 40 years, but we have never heard any of the things that he is now communicating from him before. Therefore, someone must be helping him when people were at home and giving him secretly expressions from the scriptures of the past. He is quickly learning and reciting these to other people as divine revelations during the day. This is nothing more than deception." The names of the three people who were alleged to be helping him are even mentioned in some reports. These people who were of the People of the Book were such unlettered Meccans as Addas, who was the freed slave of Huwaytib ibn Abdul Uzza, Yasar, who was the freed slave of Ala ibn al-Hadrarmi, and Jabr, who was the freed slave of Amr ibn Rabia.
The fact that the Quran directly rejected this, rather than responding to the polytheists claims can be better understood if the following points are taken into consideration:

a. The polytheists in Mecca did nothing at all to prove their allegations. They would certainly have acted if their allegations had been true. They would often have raided, for instance, the houses of the Prophet and any one whom they thought might have been helping them, seizing anything used in such a deceit to prove that the claim of prophet-hood was a "lie." It would not have been difficult for the Meccan polytheists to do such a thing, as we can see that they did not hesitate to apply or implement all sorts of actions, including torture; they were not bound by any sort of moral obligation.

b. Those who were thought to be helping the Prophet were not unknown in the city. The fact that they were uneducated was known well in Mecca. The polytheists themselves knew very well that such illiterate people could not have provided any help in producing a glorious book like the Quran, which is an extraordinary marvel of the highest literary skill. Thus, even those who did not know these emancipated slaves very well were aware that such an allegation was nonsensical and meaningless. In addition, why did such so-called helpers not claim the prophet-hood for themselves if they had such abilities?

c. Moreover, all the so-called helpers were former slaves still bound to their masters, even though they had been emancipated, according to Arabian traditions. Thus, they would not have wanted to help the Prophet in his claim as their masters would have put pressure on them with no tolerance on their existence in Mecca if they had done so.

d. Above and beyond all else, these so-called helpers all converted to Islam. Is it possible to conceive that those who helped Prophet Muhammad  would be deceived by his "trick" thus binding themselves to him? Let us even presume for a moment that they helped the Prophet, then, why was not even one of them not given a higher position or status in return for his services or help? Why were Addas, Yasar or Jabr not given higher status than Abu Bakr, Umar or Abu Ubayda?

If the prophet-hood was a deception staged with the help of these so-called assistants, we must ask why this help stayed secret even from the closest and most devoted Companions of the Prophet, for example, Ali ibn Abu Talib, Abu Bakr, or Zayd ibn Haritha. Thus, not only is such an accusation is false, God did not even deem it worthy of a reply in the Quran. Rather the Divine origin of the Quran is proclaimed:

Those who disbelieve say: "This (Quran) is but a fabrication which he (Muhammad) himself has invented, and some others have helped him with it, so they have produced a wrong and a falsehood." They also say: "(It consists of) only fables of the ancients which he has got written. They are being read to him in early mornings and evenings (while people are at home)." Say: "(It is a Book full of knowledge revealing many secrets such as no human being could in any wise discover by himself) He Who knows all the secrets contained in the heavens and the earth sends it down (to teach you some of these secrets and guide you in your life so that you may attain happiness in both worlds). He surely is All-Forgiving, All-Compassionate." (Furqan 25: 4-6)

There cannot be a worse lie than that of Quraysh polytheists, as they themselves knew much better than anyone else that such expressions were nothing more than baseless claims. The chieftains who spread such ideas among the public would certainly have been aware that the Quran, which was recited by Prophet Muhammad  could not have been a "human word" their linguistic and esthetic sensibilities were too developed not to be aware of this reality. In addition, before he was called to Prophet-hood, Muhammad was recognized as an upright and trustworthy person, one who never lied or cheated anyone. Why would such a person invent a lie against God or attribute to Him something that was not His?

But, such deceit and rumors were spread among the people; these powerful people were not only blindly stubborn, they also feared losing their social status which was connected to their being seen to be the religious leaders of society. The poor and illiterate people were not able to differentiate esthetically perfect literary works from those which were not. Hence, the status seeking chieftains tried to take advantage of the people's illiteracy, and said:

"This is but a fabrication which he himself has invented, and some others have helped him with it." (Furqan 25:4)

The polytheist Quraysh Chieftains were speaking of the three non-Arab emancipated slaves when using the word "others." The Quran goes on to speak of their nonsensical allegations with respect to the Prophet and the Revelation he received: "They also say: (It consists of) only fables of the ancients which he has got written. They are being read to him in early mornings and evening." (Furqan 25:5)

Such arguments and allegations were dependent on the stories of ancient nations found in the Quran. The Quran related stories connected to earlier nations, offering them as deterrents for the readers. The polytheists claimed that these factual stories were "fables of the ancients" recited to him secretly by these non-Arab emancipated slaves that he subsequently conveyed them to other people. They were trying to claim that Prophet Muhammad  was forging the Quran with the help, of these foreign slaves. They were well aware that such a claim was ridiculous, for how could such a book have been invented by a person, even if others were to help him? If this had been the case, then what was stopping others from inventing something similar? Why could they not make an effort like the Prophet had and invent a book similar to it, helped by others? There were plenty of people who would have been willing to back up each other with help and support. Prophet Muhammad  challenged them to do just this, but the polytheists were unable to respond to the challenge.

Excerpted from the "The Quran in 99 Questions" by Muhittin Akgul