And thou wast not (able) to recite a Book before this (Book came), nor art thou (able) to transcribe it with thy right hand: In that case, indeed, would the talkers of vanities have doubted.
That is ‘Before the revelation of the Quran you
could neither read nor write, rather you were unlettered. If it was
not so, and you were literate, then there could have been a possibility of
doubt for the infidels to put the blame that you were repeating what you
had read in the earlier books, Torah and Injil, and it was not a new
revelation sent down on you by way of prophethood.
It was a great honour and miracle for the Holy Prophet (S) to be
Allah Ta’ala had demonstrated so many evident and clear miracles to
prove the prophethood of the Holy Prophet (S), and it was one of those
miracles that He made him unlettered. Neither he could read any thing
nor could he write at all. Forty years of his life were spent in this fashion
before the eyes of the people of Makkah. He did not have any
acquaintance with the People of the Book wherefrom he could have learnt
anything. In fact, there were no People of the Book living in Makkah.
Suddenly, at the age of forty years, such a speech started flowing from his
mouth that was a miracle not only in its theme and meaning, but also in
the pinnacle of its eloquence.
Some scholars have tried to prove that he was unlettered in the
beginning, but later Allah Ta’ala taught him how to read and write. In
support of their contention they quote a hadith regarding the incident of
Hudaibiyah, which says that when the agreement between the Holy
Prophet (S) and the infidels of Makkah was being written, it was started ,by the Muslims with the words, (From Muhammad, the slave of Allah and His messenger). On this the disbelievers of Makkah
objected that his being the messenger of Allah was the real bone of
contention, and if they had accepted him as a messenger of Allah, there
would not have been any dispute among them. Therefore, they would not
accept the words ‘and His messenger’ along with his name. Sayyidna ‘Ali
(RA) had written this agreement, so, the Holy Prophet asked him to
erase it, to which he declined out of respect and reverence for him. Then
the Holy Prophet (S) took the paper in his own hand, and after erasing
the words ‘His messenger’ wrote (From Muhammad, the
son of ‘Abdullah).
In this narration, the act of writing has been attributed to the Holy
Prophet which led some people to think that he knew how to write. But
the correct position is that the act of writing was attributed to him in the
narration as a colloquial expression. It is a fairly common idiom spoken
quite widely that if someone gets something written by an ascribe, the
writing is attributed to the former, even though he has not written it
himself. The other possibility is that Allah Ta’ala made him write this bit
miraculously on this occasion. Also, by writing just a few words, one
cannot be called a literate, but would still remain an unlettered.
Furthermore, it would not be an honour for him to be labelled as literate,
rather the honour is in him being an unlettered.
Shafi, Muhammad (2008). Maariful Quran. (Vol .6 surah 29 verse 48). English-MaarifulQuran-MuftiShafiUsmaniRA-Vol-6.pdf (islamicweblibrary.com)