Allah's statement,
وإذا ءك الذين يومنون باياتنا فقل سﻼم عليكم
When those who believe in Our Ayat come to you, say: "Salamun `Alaykum'' (peace be on you);
means, honor them by returning the Salam and give them the good news of Allah's exclusive, encompassing mercy for them.
So Allah said;
كتب ربكم على نفسه الرحمة
your Lord has written Mercy for Himself,
meaning, He has obliged His Most Honored Self to grant mercy, as a favor, out of His compassion and beneficence,
أنه من عمل منكم سوءا بجهالة
So that, if any of you does evil in ignorance...
as every person who disobeys Allah does it in ignorance,
ثم تاب من بعده وأصلح
and thereafter repents and does righteous good deeds,
by repenting from the sins that he committed, intending not to repeat the sin in the future, but to perform righteous deeds,
فٲنه غفور رحيم
then surely, He is Oft-Forgiving Most Merciful.
Imam Ahmad recorded that Abu Hurayrah said that the Messenger of Allah said,
لما قضى الله الخلق كتب في كتاب فهو عنده فوق العرش إن رحمتي غلبت غضبي
When Allah finished with the creation, He wrote in a Book that He has with Him above the Throne, `My mercy prevails over My anger'.
This Hadith was also recorded in the Two Sahihs