Ibn Jarir commented on the Ayah, "The Ayah means, this is a Book that We sent down, so that you do not say,
إِنَّمَآ أُنزِلَ الْكِتَـبُ عَلَى طَآئِفَتَيْنِ مِن قَبْلِنَا
("The Book was sent down only to two sects before us.") This way, you will have no excuse. Allah said in another Ayah,
وَلَوْلا أَن تُصِيبَهُم مُّصِيبَةٌ بِمَا قَدَّمَتْ أَيْدِيهِمْ فَيَقُولُواْ رَبَّنَا لَوْلا أَرْسَلْتَ إِلَيْنَا رَسُولاً فَنَتِّبِعَ ءايَـتِكَ
(Otherwise, they would have suffered a calamity because of what their hands sent forth, and said: "Our Lord! Why did You not send us a Messenger We would then have followed Your Ayat.")28:47." The Ayah,
عَلَى طَآئِفَتَيْنِ مِن قَبْلِنَا
(to two sects before us) refers to the Jews and Christians, according to `Ali bin Abi Talhah who narrated it from Ibn `Abbas. Similar was reported from Mujahid, As-Suddi, Qatadah and several others. Allah's statement,
وَإِن كُنَّا عَن دِرَاسَتِهِمْ لَغَـفِلِينَ
("...and for our part, we were in fact unaware of what they studied.") meaning: `we did not understand what they said because the revelation was not in our tongue. We, indeed, were busy and unaware of their message,' so they said. Allah said next,
أَوْ تَقُولُواْ لَوْ أَنَّآ أُنزِلَ عَلَيْنَا الْكِتَـبُ لَكُنَّآ أَهْدَى مِنْهُمْ
(Or lest you should say: "If only the Book had been sent down to us, we would surely, have been better guided than they.") meaning: We also refuted this excuse, had you used it, lest you say, "If a Book was revealed to us, just as they received a Book, we would have been better guided than they are." Allah also said.
وَأَقْسَمُواْ بِاللَّهِ جَهْدَ أَيْمَـنِهِمْ لَئِن جَآءَهُمْ نَذِيرٌ لَّيَكُونُنَّ أَهْدَى مِنْ إِحْدَى الاٍّمَمِ
(And they swore by Allah their most binding oath that if a warner came to them, they would be more guided than any of the nations (before them).) 35:42 Allah replied here,
فَقَدْ جَآءَكُمْ بَيِّنَةٌ مِّن رَّبِّكُمْ وَهُدًى وَرَحْمَةٌ
(So now has come unto you a clear proof from your Lord, and a guidance and a mercy.) Allah says, there has come to you from Allah a Glorious Qur'an revealed to Muhammad ﷺ, the Arab Prophet. In it is the explanation of the lawful and unlawful matters, guidance for the hearts and mercy from Allah to His servants who follow and implement it. Allah said;
فَمَنْ أَظْلَمُ مِمَّن كَذَّبَ بِآيَـتِ اللَّهِ وَصَدَفَ عَنْهَا
(Who then does more wrong than one who rejects the Ayat of Allah and Sadafa away therefrom) This refers to the one who neither benefited from what the Messenger brought, nor followed what he was sent with by abandoning all other ways. Rather, he Sadafa from following the Ayat of Allah, meaning, he discouraged and hindered people from following it. This is the explanation of As-Suddi for Sadafa, while Ibn `Abbas, Mujahid and Qatadah said that Sadafa means, he turned away from it.