"So if you are in doubt, [O Muhammad], about that which We have revealed to you, then ask those who have been reading the Scripture before you," meaning: those of them who believed.
﴿فَإِنْ كُنْتَ فِي شكٍّ مِمَّا أَنْزَلْنَا إِلَيْكَ فَاسْأَلِ الَّذِينَ يَقْرَءُونَ الْكتاب من قبلك﴾ يَعْنِي: مَنْ آمَنَ مِنْهُمْ.