In rejection of the idolators' questioning about the Day of Judgement, due to their denial of its occurrence, Allah says,

﴿عَمَّ يَتَسَآءَلُونَ - عَنِ النَّبَإِ الْعَظِيمِ ﴾

(What are they asking about About the great news,) meaning, what are they asking about They are asking about the matter of the Day of Judgement, and it is the great news. Meaning the dreadful, horrifying, overwhelming information.

﴿الَّذِى هُمْ فِيهِ مُخْتَلِفُونَ ﴾

(About which they are in disagreement.) meaning, the people are divided into two ideas about it. There are those who believe in it and those who disbelieve in it. Then Allah threatens those who deny the Day of Judgement by saying,

﴿كَلاَّ سَيَعْلَمُونَ - ثُمَّ كَلاَّ سَيَعْلَمُونَ ﴾

(Nay, they will come to know! Nay, again, they will come to know!) This is a severe threat and a direct warning.