( الَّذِينَ اتَّخَذُوا دِينَهُمْ لَهْوًا وَلَعِبًا) (Who took their religion as distraction and amusement): The people who used to make fun of religion and fought religious people.

( وَغَرَّتْهُمُ الْحَيَاةُ الدُّنْيَا ) (and whom the worldly life deluded): They thought that the worldly life is the ultimate goal of living and that was their mistake, for there are both the worldly life and the Hereafter. Just like Abdullah bin Amr bin Al-Aas (may Allah be pleased with them) said:

“Work for this life as if you will live forever, and work for the Hereafter as if you will die tomorrow.”

Allah (swt) says:

 وَابْتَغِ فِيمَا آتَاكَ اللَّهُ الدَّارَ الْآخِرَةَ ۖ وَلَا تَنسَ نَصِيبَكَ مِنَ الدُّنْيَا ۖ وَأَحْسِن كَمَا أَحْسَنَ اللَّهُ إِلَيْكَ ۖ وَلَا تَبْغِ الْفَسَادَ فِي الْأَرْضِ ۖ إِنَّ اللَّهَ لَا يُحِبُّ الْمُفْسِدِينَ

(But seek, through that which Allah has given you, the home of the Hereafter; and [yet], do not forget your share of the world. And do good as Allah has done good to you. And desire not corruption in the land. Indeed, Allah does not like corrupters.”) (Al-Qasas: 77)

( فَالْيَوْمَ نَنسَاهُمْ كَمَا نَسُوا لِقَاءَ يَوْمِهِمْ هَٰذَا) (So today We will forget them just as they forgot the meeting of this Day of theirs): To be forgotten means that they will be rejected by Allah (swt), He (swt) will not turn to them, nor will His Mercy (swt) reach them because they forgot the meeting on that Day, even though Messengers were sent to warn them of it and Books were revealed upon them as well.

( وَمَا كَانُوا بِآيَاتِنَا يَجْحَدُونَ) (and for having rejected Our verses): For disbelieving and denying Allah’s (swt) verses, and for being unrightfully arrogant towards Allah (swt).