Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(And we have revealed the Scripture unto thee) We did send Gabriel to you with the Qur'an (only that thou mayst explain unto them that wherein) in Religion (they differ) they opposed, (and (as) a guidance) from error (and a mercy) from chastisement (for a people who believe) in it.
And We have not revealed to you, O Muhammad (s), the Book, the Qur’ān, except [for the purpose] that you may make clear to them, to mankind, that wherein they differ, in the matter of religion, and as a guidance (wa-hudan is a supplement to wa-li-tubayyina, ‘and that you may make clear’) and as a mercy for a people who believe, in it.
Finding Consolation in the Reminder of Those Who came before
Allah says, `He sent Messengers to the nations of the past, and they were rejected. You, O Muhammad, have an example in your brothers among the Messengers, so do not be distressed by your people's rejection. As for the idolators' rejection of the Messengers, the reason for this is that the Shaytan made their deeds attractive to them.'
فَهُوَ وَلِيُّهُمُ الْيَوْمَ
(So today he (Shaytan) is their helper,) meaning they will be suffering punishment while Shaytan is their only helper, and he cannot save them, so they have no one to answer their calls for help, and theirs is a painful punishment.
The Reason why the Qur'an was revealed
Then Allah says to His Messenger that He has revealed the Book to him to explain the truth to mankind in matters which they dispute over. So the Qur'an is a decisive arbitrator for every issue that they argue about.
(and (as) a guidance) meaning, for their hearts.
(and a mercy) meaning, for the one who adheres to it.
(for a people who believe.) Just as Allah causes the Qur'an to bring life to hearts that were dead from disbelief, so He brings the earth to life after it has died, by sending down water from the sky.
إِنَّ فِى ذلِكَ لآيَةً لِقَوْمٍ يَسْمَعُونَ
(Surely that is a sign for people who listen.) meaning those who understand the words and their meanings.
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
That is, "This Book has afforded them an excellent opportunity to resolve their differences which have been caused by the superstitious creeds of their forefathers, and which have divided, them into warring factions. They can then unite on the permanent basis of the Truth presented by the Qur'an, but those foolish people preferring their former condition even after the coming of this blessing, shall meet with disgrace and torment. On the other hand, only those, who believe in this Book, will find the Right Way and blessed with mercy and favors from Allah.”