The Conclusion
Allah, the Exalted, is saying, "We relate all of these stories to you (Muhammad) concerning what happened with the Messengers who came before you with their nations. This is an explanation of what transpired in their arguments and disputes and how the Prophets were all rejected and harmed. These stories also explain how Allah helped His party of believers and disgraced His enemies, the disbelievers. We relate all of this to you (Muhammad) in order to make your heart firm and so that you may take an example from your brothers who passed before you of the Messengers."
Concerning Allah's statement,
{And in this has come to you the truth,}

This is referring to this Surah itself. This was said by Ibn 'Abbas, Mujahid and a group of the Salaf and it is the correct view. This means, This comprehensive Surah contains the stories of the Prophets and how Allah saved them, and the believers along with them and how He destroyed the disbelievers. There has come to you (Muhammad) stories of...