"Nay, if you knew with the knowledge of certainty." That is: your knowledge is not the knowledge of certainty, meaning the polytheists, and that the knowledge of the believers is the knowledge of certainty.
﴿كَلَّا لَوْ تَعْلَمُونَ عِلْمَ الْيَقِين﴾ أَيْ: أَنَّ عِلْمَكُمْ لَيْسَ بِعِلْمِ الْيَقِينِ يَعْنِي: الْمُشْرِكِينَ وَأَنَّ عِلْمَ الْمُؤمنِينَ هُوَ علم الْيَقِين
