﴿ وَإِن مِّنْ أَهْلِ ٱلْكِتَٰبِ إِلَّا لَيُؤْمِنَنَّ بِهِۦ قَبْلَ مَوْتِهِۦ ۖ وَيَوْمَ ٱلْقِيَٰمَةِ يَكُونُ عَلَيْهِمْ شَهِيدًۭا ﴾

“Yet there is not one of the followers of earlier revelation who does not, at the moment of his death, grasp the truth about Jesus; and on the Day of Resurrection he [himself] shall bear witness to the truth against them.”

[he will be] a witness, witnessing them, the Truth disclosing Himself to them in his form, as has been alluded to.