﴿ ثُمَّ كَلَّا سَيَعْلَمُونَ ﴾

“And once again: Nay, but in time they will come to understand!”

(Nay) truly, (again, but they will come to know!) They will come to know in the grave what will be done with them. This is a threat from Allah to those who disbelieved in Muhammad (pbuh) and in the Qur'an.