Allah said,
وَالَّذِينَ ءَاتَيْنَـهُمُ الْكِتَـبَ
(Those to whom We have given the Book,) and they adhere by it,
يَفْرَحُونَ بِمَآ أُنزِلَ إِلَيْكَ
(rejoice at what has been revealed unto you,) i.e. the Qur'an, because they have evidence in their Books affirming the truth of the Qur'an and conveying the good news of its imminent revelation, just as Allah said in another Ayah,
الَّذِينَ آتَيْنَـهُمُ الْكِتَـبَ يَتْلُونَهُ حَقَّ تِلاَوَتِهِ
(Those to whom We gave the Book recite it as it should be recited.) 2:121 Allah said,
قُلْ ءَامِنُواْ بِهِ أَوْ لاَ تُؤْمِنُواْ
(Say: "Believe in it (the Qur'an) or do not believe.") 17:107, until,
إِن كَانَ وَعْدُ رَبِّنَا لَمَفْعُولاً
(Truly, the promise of our Lord must be fulfilled.) 17:109 meaning, Allah's promise to us in our Books to send Muhammad ﷺ is true. It is certain and will surely come to pass and be fulfilled, so all praise to our Lord, how truthful is His promise, all the thanks are due to Him,
وَيَخِرُّونَ لِلاٌّذْقَانِ يَبْكُونَ وَيَزِيدُهُمْ خُشُوعًا
(And they fall down on their faces weeping and it increases their humility.) 17:109 Allah said next,
وَمِنَ الاٌّحْزَابِ مَن يُنكِرُ بَعْضَهُ
(but there are among the Ahzab (Confederates) those who reject a part thereof.) meaning, `There are those among the sects who disbelieve in some of what was revealed to you (O Muhammad).' Mujahid said that,
وَمِنَ الاٌّحْزَابِ
(but there are among the Ahzab (Confederates)), refers to Jews and Christians,
مَن يُنكِرُ بَعْضَهُ
(those who reject a part thereof), meaning, `They reject a part of the truth that came down to you - O Muhammad.' Similar was reported from Qatadah and `Abdur-Rahman bin Zayd bin Aslam. Allah said in similar Ayat,
وَإِنَّ مِنْ أَهْلِ الْكِتَـبِ لَمَن يُؤْمِنُ بِاللَّهِ
(And there are, certainly, among the People of the Scripture, those who believe in Allah.) 3:199 Allah said next,
قُلْ إِنَّمَآ أُمِرْتُ أَنْ أَعْبُدَ اللَّهَ وَلا أُشْرِكَ بِهِ
(Say: "I am commanded only to worship Allah and not to join partners with Him...") meaning, `I (Muhammad) was sent with the religion of worshipping Allah alone without partners, just as the Messengers before me,
إِلَيْهِ أَدْعُو
(To Him (alone) I call), I call the people to His path,
وَإِلَيْهِ مَآبِ
(and to Him is my return.) final destination and destiny.' Allah said,
وَكَذلِكَ أَنزَلْنَاهُ حُكْمًا عَرَبِيًّا
(And thus have We sent it (the Qur'an) down to be a judgement of authority in Arabic.) Allah says, `Just as We sent Messengers before you and revealed to them Divine Books from heaven, We sent down to you the Qur'an, a judgement of authority in Arabic, as an honor for you, and We preferred you among all people with this clear, plain and unequivocal Book that,
لاَّ يَأْتِيهِ الْبَـطِلُ مِن بَيْنِ يَدَيْهِ وَلاَ مِنْ خَلْفِهِ تَنزِيلٌ مِّنْ حَكِيمٍ حَمِيدٍ
(Falsehood cannot come to it from before it or behind it: (it is) sent down by the All-Wise, Worthy of all praise.)' 41:42 Allah's statement,
وَلَئِنِ اتَّبَعْتَ أَهْوَآءَهُم
(Were you to follow their (vain) desires), means, their opinions,
بَعْدِ مَا جَآءَكَ مِنَ الْعِلْمِ
(after the knowledge which has come to you) from Allah, all praise to Him,
مَا لَكَ مِنَ اللَّهِ مِن وَلِىٍّ وَلاَ وَاقٍ
(then you will not have any Wali (protector) or defender against Allah.) This part of the Ayah warns people of know- ledge against following the paths of misguid- ance after they had gained knowledge in (and abided by) the Prophetic Sunnah and the path of Muhammad ﷺ, may Allah's best peace and blessings be on him.