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.'
(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.
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.
(Surely that is a sign for people who listen.) meaning those who understand the words and their meanings.