All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Shaykh Al-Islam Ibn Taymiyyah said in Daqaa’iq at-Tafseer: “In conclusion, we say that the pronouncement that the People of the Book who are mentioned in the Quran are those whose grandfathers entered that religion before it was abrogated (by Prophet Muhammad ) is a weak statement, and the pronouncement that ‘Ali ibn Abi Taalib implied this is also weak. Rather, the correct opinion that is absolute is that the man being from the People of the Book or not is an independent ruling and everyone who adopts the religion of the People of the Book is one of them, whether his father or grandfather embraced their religion or did not embrace it, and whether he embraced it before the ruling was abrogated or after it was abrogated. This is the view of the of the majority of the scholars such as Abu Haneefah and Maalik, and it is the explicit view of Ahmad although there is a well-known difference of opinion about it among Ahmad's companions. This view is also the adopted view among the Companions (of the Prophet, sallallaahu ‘alayhi wa sallam) and I do not know any difference of opinion among the Companions in this regard.”
Allaah Knows best.