﴿ ٱلَّذِى هُمْ فِيهِ مُخْتَلِفُونَ ﴾

“on which they [so utterly] disagree.”

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.