هُوَ الَّذِي بَعَثَ فِي الاُْمِّيِّينَ رَسُولاً مِّنْهُمْ
He it is Who sent among the unlettered ones a Messenger from among themselves,
the word `unlettered' here refers to the Arabs.
Allah the Exalted said in another Ayah,
وَقُلْ لِّلَّذِينَ أُوتُواْ الْكِتَـبَ وَالاٍّمِّيِّينَ ءَأَسْلَمْتُمْ فَإِنْ أَسْلَمُواْ فَقَدِ اهْتَدَواْ وَّإِن تَوَلَّوْاْ فَإِنَّمَا عَلَيْكَ الْبَلَـغُ وَاللَّهُ بَصِيرٌ بِالْعِبَادِ
And say to those who were given the Scripture and those who are illiterates;"Do you submit yourselves If they do, they are rightly guided; but if they turn away, your duty is only to convey the message; and Allah is All-Seer of (His) servants." (3;20)
Mentioning the unlettered ones in specific here does not mean that Muhammad was only sent to them, because the blessing to the Arabs is greater than that of other nations.
In another Ayah, Allah said,
وَإِنَّهُ لَذِكْرٌ لَّكَ وَلِقَوْمِكَ
And verily, this is indeed a Reminder for you and your people. (43;44)
Surely, the Qur'an is also a reminder for those other than Arabs to take heed.
Allah the Exalted said,
وَأَنذِرْ عَشِيرَتَكَ الاٌّقْرَبِينَ
And warn your tribe of near kindred. (26;214)
These Ayat do not negate Allah's statements,
قُلْ يَأَيُّهَا النَّاسُ إِنِّى رَسُولُ اللَّهِ إِلَيْكُمْ جَمِيعًا
Say;"O mankind! verily, I am sent to you all as the Messenger of Allah. (7;158)
and,
لااٌّنذِرَكُمْ بِهِ وَمَن بَلَغَ
that I may therewith warn you and whomsoever it may reach. (6;19)
and in His statement about the Qur'an,
وَمَن يَكْفُرْ بِهِ مِنَ الاٌّحْزَابِ فَالنَّارُ مَوْعِدُهُ
But those of the groups that reject it, the Fire will be their promised meeting place. (11;17)
There are other Ayat that indicate that his Message is universal. He, may Allah's peace and blessings be upon him, was sent to all people, mankind and the Jinns alike. We mentioned this meaning before in Surah Al-An`am producing various Ayat and Hadiths. All praise and thanks are to due to Allah.
This Ayah testifies that Allah has indeed accepted the invocation of His friend Ibrahim when he supplicated Allah to send a Messenger to the people of Makkah from among them their own. One who will recite to them Allah's statements, purify them and teach them the Book and the Hikmah. So, Allah -- all praise and thanks be to Him -- sent him when the Messengers ceased and the way was obscure. Indeed it was a time when it was most needed. Especially since Allah hated the people of the earth, Arabs and non-Arabs alike, except for a few of the People of the Scripture, who kept to the true faith Allah the Exalted sent to `Isa bin Maryam, peace be upon him. This is why Allah said,
هُوَ الَّذِي بَعَثَ فِي الاُْمِّيِّينَ رَسُولاً مِّنْهُمْ يَتْلُو عَلَيْهِمْ ايَاتِهِ وَيُزَكِّيهِمْ وَيُعَلِّمُهُمُ الْكِتَابَ وَالْحِكْمَةَ
وَإِن كَانُوا مِن قَبْلُ لَفِي ضَلَإلٍ مُّبِينٍ
He it is Who sent among the unlettered ones a Messenger from among themselves, reciting to them His Ayat, purifying them, and teaching them the Book and the Hikmah. And verily, they had been before in manifest error.
In ancient times, the Arabs used to adhere by the religion of Ibrahim, peace be upon him. They later changed, corrupted and contradicted it, choosing polytheism instead of Tawhid and doubts instead of certainty. They invented a religion that Allah did not legislate, just as the People of the Scriptures did when they changed and corrupted their Divine Books. Allah sent Muhammad, with a great divine legislation, perfect religion that is suitable for all humans and Jinns.
In it, there is guidance and explanations of all that they need in this life and the Hereafter. It draws them closer to Paradise and Allah's pleasure and takes them away from the Fire and earning Allah's anger.
In it, there is the final judgement for all types of doubts and suspicion for all major and minor matters of the religion.
In Muhammad, Allah gathered all the good qualities of the Prophets before him, and gave him what He has never given the earlier and later generations of mankind.
May Allah's peace and blessings be on Muhammad until the Day of Judgement.