﴿ لَا يُؤْمِنُونَ بِهِۦ حَتَّىٰ يَرَوُا۟ ٱلْعَذَابَ ٱلْأَلِيمَ ﴾

“they will not believe in it till they behold the grievous suffering”

(They will not believe in it) so that they do not believe in Muhammad (pbuh) or in the Qur'an (till they behold the painful doom,