﴿ ٱلَّذِى هُمْ فِيهِ مُخْتَلِفُونَ ﴾

“on which they [so utterly] disagree.”

(Concerning which they are in disagreement) some of them disbelieve in Muhammad (pbuh) and in the Qur'an while others believe in them. This is because whenever Gabriel brought down a section of the Qur'an to the Prophet (pbuh) and he recited it to them, they used to talk about it, and some of them would believe while others would persist in their disbelief.