Bukhari

Hadith Commentary:
The conditions that Islam has set for a trustworthy and upright witness are such that it is impossible for a non-Muslim to meet their standard. Therefore, as a general rule, the testimony of a non-Muslim is not acceptable. Imam al-Bukhari rahimahullah is presenting the evidences for this position. However, it is a separate matter that the Imam of the time or an authorized judge may accept the testimony of a non-Muslim on the basis that it is corroborated by some other reliable indications. For example, the Prophet sallallahu alayhi wa sallam himself ordered the stoning of a Jewish man and a Jewish woman for the crime of adultery based on the testimony of four Jews. In any case, the general principle remains as stated by the Imam.

Hadith Commentary:
(1)
The conditions that Islam has set for a trustworthy and upright witness cannot be fulfilled by a non-Muslim according to their standards; therefore, the testimony of any disbeliever or polytheist is not accepted. Imam Bukhari rahimahullah has provided strong and solid proofs to establish this position. It is a separate matter if a ruler of the time accepts the testimony of a non-Muslim on the basis that it is corroborated by other reliable sources, as the Messenger of Allah sallallahu alayhi wa sallam himself stoned a Jewish man and woman for the crime of adultery based on the testimony of four Jews. However, the established rule is the one stated by Imam Bukhari rahimahullah: the testimony of disbelievers and polytheists is inadmissible.

(2)
In the aforementioned hadith, there is a prohibition against asking the People of the Book. Since they altered the books sent by Allah, their reports are not to be accepted. If a person's report is not accepted, then how can his testimony be taken? Therefore, the testimony of the People of the Book is not accepted. (Fath al-Bari: 5/359)

Hadith Commentary:
You have with you the true word of Allah, the Noble Qur’an, and its explanation, the hadith, is also with you. Then it is a matter of great shame that you ask them (the People of the Book). Many scholars, based on this hadith, have even considered it disliked (makruh) to study the Gospel (Injil) and previous heavenly scriptures, because they have been altered and changed.
Lest it happen
that the belief (‘aqidah) of those with weak faith becomes corrupted. However, for the person who does not have this fear, and who wishes to debate with the People of the Book and respond to the objections they raise against Islam, then for him it is not disliked (makruh); rather, it is even a source of reward.
“Innamal a‘malu bin-niyyat” (Actions are only by intentions).

Hadith Commentary:
➊
The intent of Ibn Abbas radi Allahu anhu is that the People of the Book do not ask you about your religion, so what affliction has befallen you that you go around asking them?
You have the true word of Allah in the form of the Qur’an.
Then, in the form of hadith, you have its explanation with you as well, so it is a matter of great shame that you ask them.
➋
Many scholars, in light of this hadith, have declared it disliked (makruh) to study the Torah, the Gospel, and other religious books, lest the beliefs of those with weak faith become further corrupted, because these books have been altered and changed. However, for the person who does not have this fear, and who needs to debate with the People of the Book and respond to the objections raised against Islam, then for such a person, studying the Bible is not disliked (makruh); rather, it is hoped that it will be a means of reward and recompense with Allah.
And Allah knows best.