Islam4You

Ibn ʿAbbās (Allah be pleased with him) narrated that there were some people who had killed a lot of people unjustly and had committed a lot of shameful acts. They came to the Messenger of Allāh ﷺ and submitted before him, 'The religion (Islām) you (ﷺ) invite us to is, no doubt, very good, but we have a problem. We think about all those major sins we have already committed. Now, if we were to become Muslims, do you (ﷺ) have hope that our repentance will be accepted?' - thereupon Allāh revealed the verse:

قُلْ يَا عِبَادِيَ الَّذِيْنَ أَسْرَفُوْا عَلَىٰ أَنفُسِهِمْ لَا تَقْنَطُوْا مِن رَّحْمَةِ اللهِ ۚ إِنَّ اللهَ يَغْفِرُ الذُّنُوْبَ جَمِيْعًا ۚ إِنَّهُ هُوَ الْغَفُوْرُ الرَّحِيْمُ

Say, (O Prophet ﷺ, that Allāh says), “O My servants who have exceeded the limits against their souls! Do not lose hope in Allāh’s mercy, for Allāh certainly forgives all sins. He is indeed the All-Forgiving, Most Merciful."
[Qurʾān, Sūrah az-Zumar (39), verse 53]

Well before death arrives, sincere repentance made from sins, no matter how serious, even from kufr (disbelief) and shirk (ascribing partners to Allāh) gets accepted. And by the virtue of genuine repentance, all sins will be forgiven (inshāʾAllāh) and no one should lose hope in the mercy of Allāh.

ʿAbdullāh Ibn ʿUmar (Allah be pleased with him) said that out of all the verses of the Qurʾān, this verse (39:53) brings the strongest message of hope for sinners.

[Maʿāriful Qurʾān, Volume 7, Page 573-574]