عَمَّ یَتَسَآءَلُوْنَ ۟ۚعَمَّ يَتَسَآءَلُوۡنَۚ
What are they asking each other about?
— Tazkirul Quran (En)
عَنِ النَّبَاِ الْعَظِیْمِ ۟ۙعَنِ النَّبَاِ الۡعَظِيۡمِۙ
About the awesome tidings [of resurrection]
— Tazkirul Quran (En)
الَّذِیْ هُمْ فِیْهِ مُخْتَلِفُوْنَ ۟ؕالَّذِىۡ هُمۡ فِيۡهِ مُخۡتَلِفُوۡنَؕ
concerning which they are in disagreement!
— Tazkirul Quran (En)
Tafseer of 78:1–3
The Arab people were not disbelievers in the Hereafter. But they did not believe in a Hereafter of such a nature as is mentioned in the Quran. In other words, they doubted that by rejecting Muhammad, they would face degradation and humiliation in the world of the Hereafter.