﴿ وَبِكُفْرِهِمْ وَقَوْلِهِمْ عَلَىٰ مَرْيَمَ بُهْتَٰنًا عَظِيمًۭا ﴾

“and for their refusal to acknowledge the truth, and the awesome calumny which they utter against Mary,”

And for their disbelief a second time in Jesus the bā’ of bi-kufrihim ‘for their disbelief’ is repeated in order to separate it ‘their disbelief’ in Jesus from what is supplemented to it ‘their utterance against Mary’ and their uttering against Mary a tremendous calumny when they accused her of fornication.