(… رَبَّنَا إِنَّكَ جَامِعُ النَّاسِ لِيَوْمٍ لَّا رَيْبَ فِيهِ ) (Our Lord, surely You will gather the people for a Day about which there is no doubt …): Every person who considers this life only sees only an act of the play of life. The latter, however, has two acts: earthly life and the afterlife. So, you have a blurry image if you neglect the second act. The base for one’s deeds is that the Almighty brings people back in a day on which there is no doubt; i.e. the doomsday, the day of reckoning before Allah (swt):
وَيَوْمَ يَعَضُّ الظَّالِمُ عَلَىٰ يَدَيْهِ يَقُولُ يَا لَيْتَنِي اتَّخَذْتُ مَعَ الرَّسُولِ سَبِيلً * يَا وَيْلَتَىٰ لَيْتَنِي لَمْ أَتَّخِذْ فُلَانًا خَلِيلًا * لَّقَدْ أَضَلَّنِي عَنِ الذِّكْرِ بَعْدَ إِذْ جَاءَنِي ۗ وَكَانَ الشَّيْطَانُ لِلْإِنسَانِ خَذُولًا
(And the Day the wrongdoer will bite on his hands [in regret] he will say, “Oh, I wish I had taken with the Messenger a way.% Oh, woe to me! I wish I had not taken that one as a friend.% He led me away from the remembrance after it had come to me. And ever is Satan, to man, a deserter.”) (Al-Furqaan: 27-29)
The believers keep remembering, in their supplication, that the Almighty will bring people altogether back in the Day of Resurrection, settle and judge among them in all what they have had disagreement upon, and reward everyone according to their deeds in the earthly life, be it good or bad.
( إِنَّ اللَّهَ لَا يُخْلِفُ الْمِيعَادَ … ) (…Allah does not fail in His promise): When the Almighty promises something, such promise is an absolute reality.