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 none 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;” That is: 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).” It is the Qur’aan that gives light 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;’’ That is: 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).”
[Slightly paraphrased..for further details see Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan’ by Imaam Sa’di (rahimahullaah)]