In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Allaah [The Most High] said:
هُوَ الَّذِي بَعَثَ فِي الْأُمِّيِّينَ رَسُولًا مِّنْهُمْ
He it is Who sent among the unlettered ones a Messenger (Muhammad) from among themselves. [62:2]
Imaam Ibn Rajab [rahimahullaah] said:
The fact that he is from them carries two beneficial [reminders]: Firstly, indeed this Messenger was also unlettered just like the unlettered ones he was sent to. He never read a book nor wrote with his hand, as Allaah [The Most High] said:
وَمَا كُنتَ تَتْلُو مِن قَبْلِهِ مِن كِتَابٍ وَلَا تَخُطُّهُ بِيَمِينِكَ
Neither did you (O Muhammad) read any book before it (this Qur’an), nor did you write any book (whatsoever) with your right hand. [29:48]
He never left the land of his people [to reside elsewhere] in order to learn anything from them; rather he did not cease being unlettered among the unlettered nation. He neither wrote nor read until he reached the age of forty, then he was given this manifest Book [the Qur’an] that makes things clear.
Secondly: A notification that the one sent among them- the unlettered ones of Makkah in particular- knew his lineage, nobility, truthfulness, trustworthiness and chastity. He grew up among them whilst being well known for [these traits] and that he has never lied. Therefore, when it is the fact that he refrained from lying about the people, then how can he fabricate lies about Allaah. [Ref 1]
Allaah (The Most High) said:
يَـٰٓأَهۡلَ ٱلۡڪِتَـٰبِ قَدۡ جَآءَڪُمۡ رَسُولُنَا يُبَيِّنُ لَكُمۡ ڪَثِيرً۬ا مِّمَّا ڪُنتُمۡ تُخۡفُونَ مِنَ ٱلۡڪِتَـٰبِ وَيَعۡفُواْ عَن ڪَثِيرٍ۬ۚ قَدۡ جَآءَڪُم مِّنَ ٱللَّهِ نُورٌ۬ وَڪِتَـٰبٌ۬ مُّبِينٌ۬
يَهۡدِى بِهِ ٱللَّهُ مَنِ ٱتَّبَعَ رِضۡوَٲنَهُ ۥ سُبُلَ ٱلسَّلَـٰمِ وَيُخۡرِجُهُم مِّنَ ٱلظُّلُمَـٰتِ إِلَى ٱلنُّورِ بِإِذۡنِهِۦ وَيَهۡدِيهِمۡ إِلَىٰ صِرَٲطٍ۬ مُّسۡتَقِيمٍ۬
O people of the Scripture (Jews and Christians)! Now has come to you Our Messenger (Muhammad) explaining to you much of that which you used to hide from the Scripture and passing over (i.e. leaving out without explaining) much. Indeed, there has come to you from Allah a light (Prophet Muhammad) and a plain Book (this Qur’an). Wherewith Allah guides all those who seek His Good Pleasure to ways of peace, and He brings them out of darkness by His Will unto light and guides them to a Straight Way (Islamic Monotheism). [Surah Al-Maa-idah Aayaat 15-16]
After Allaah made a mention of the covenant He took from the people of the scripture-the Jews and Christians- [i.e. in Aayaat 12-14 Surah Maa-idah] and that they violated it except a few amongst them, He then commanded all of them to believe in Muhammad (sallal-laahu – alayhi – wasallam). He established the proof against them through clear signs that shows the soundness of Muhammad’s (sallal-laahu-alayhi-wasallam) Prophethood; and that is: He explained to them much of that which they concealed from mankind, even from the common people who follow their religion.
And if they [i.e. the Jews and Christians] were the ones with knowledge and no one else possessed knowledge during that era except them, then an eager seeker of knowledge would not have had any access to it, except through them. So the Messenger’s arrival with this Great Qur’aan that clarifies for them that which they used to hide amongst themselves–whilst he was unlettered and could neither read nor write–is from the surest of evidences concerning the unequivocal establishment of his Messenger ship, and likewise his qualities mentioned in their books, the presence of the prophecies about him in their books, the verses concerning the divine legislated laws against adultery and what is similar to it.
[وَيَعۡفُواْ عَن ڪَثِيرٍ -And passing over (i.e. leaving out without explaining) much]- Meaning: He abstained from explaining that which is not in agreement with the perfect and infallible wisdom contained in the divine laws. [قَدۡ جَآءَڪُم مِّنَ ٱللَّهِ نُورٌ۬ وَڪِتَـٰبٌ۬ مُّبِينٌ۬- Indeed, there has come to you from Allah a light (Prophet Muhammad) and a plain Book (this Qur’an)]- Meaning: The Qur’aan that gives light [to those] in the midst of the darkness of ignorance and the foolishness of misguidance. [وَڪِتَـٰبٌ۬ مُّبِينٌ۬ – And a plain Book (this Qur’an)], for everything needed by creation [i.e. Mankind and Jinn] in their worldly and religious affairs–such as knowledge and awareness of Allaah; His Names, Attributes and Actions; knowledge of the rulings of the divine Islamic legislation and His Rulings on recompense.
Then He (The Most High ) mentioned those who are guided by this Qur’aan and the reason behind the slave being guided, so He (The Most High ) said : [يَهۡدِى بِهِ ٱللَّهُ مَنِ ٱتَّبَعَ رِضۡوَٲنَهُ ۥ سُبُلَ ٱلسَّلَـٰمِ -Wherewith Allah guides all those who seek His Good Pleasure to ways of peace]- Meaning: It guides the one who strives and is eager to seek Allaah’s Good pleasure with a good aim; (to the) [سُبُلَ ٱلسَّلَـٰمِ -Ways of peace] that safeguards a person from punishment when he follows them and leads him to the abode of peace (i.e. paradise), and that is to possess knowledge of the truth and to act upon it- in its general and specific affairs. [وَيُخۡرِجُهُم مِّنَ – And He brings them out] from the darkness of disbelief, bidah, disobedience, ignorance and heedlessness, and enters them into the light of Imaan, the Sunnah, obedience, knowledge and remembrance of Allaah. All these affairs of guidance are reached by the permission of Allaah- The One that whatever He wills has to take place and whatever He does not will cannot take place. [وَيَهۡدِيهِمۡ إِلَىٰ صِرَٲطٍ۬ مُّسۡتَقِيمٍ – And guides them to a Straight Way (Islamic Monotheism)]. [Ref 2]
[Ref 1: Source: An Excerpt from Lataa’if Al-Ma’aarif. Page 166-16. slightly paraphrased]; [Ref 2: An Excerpt from Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan’ by Imaam Sa’di (rahimahullaah). Slightly paraphrased