From the evidence I’ve seen, I don’t personally believe Muhammad himself believed the scriptures had been corrupt. The proof texts Muslims use from the Quran are highly controversial and ambiguous.
Muhammad has praised the scriptures many times in the Quran; you’ll never find alongside these verses of praise a verse saying “but unfortunately they are now corrupt”, the scriptures are praised full stop…I hope our Muslim friends start using thinking about these sort of things
I think you’re right about the Qur’an itself, that its treatment of Jewish and Christian scriptures is usually positive and, at worst, ambiguous.
Here’s one of those “full stop” texts you mention:
Quran:
God, (there is) no god but He, the Everliving, the Self-subsisting by Whom all things subsist. He has revealed to you the Book with truth, verifying that which is before it, and He revealed the Torah and the Gospel aforetime, a guidance for the people, and He sent the Furqan. Surely they who disbelieve in the communications of God they shall have a severe chastisement; and God is Mighty, the Lord of retribution.
As you say, there’s no “God revealed the Torah and the Gospel, but you can’t follow them anymore” qualification.
Sometimes the Qur’an accuses Jews and Christians of not obeying what their own scriptures teach, forgetting or misunderstanding what their own scriptures teach, or of perverting the meaning of those scriptures. All of these accusations are based on the implicit assumption that there is
nothing wrong with the scriptures Jews and Christians already have, because it would make little sense to rebuke someone for disobeying, ignoring, or misunderstanding something one doesn’t believe to be true in the first place.
In at least one place, the Qur’an accuses some
unspecified group of Jews, Christians, and/or
Sabians of forgery for profit: “Woe, then, to those who write the book with their hands and then say: This is from God, so that they may take for it a small price; therefore woe to them for what their hands have written and woe to them for what they earn.” This passage does not, however, support the broader idea that all Jewish and Christian scriptures have been forged/corrupted or that all Jews and Christians are using forged/corrupted scriptures. The Qur’an itself does not say that.
From what I’ve seen, the now common Muslim belief that all Jewish and Christian scriptures have been forged/corrupted, or that all Jews and Christians are using forged/corrupted scriptures, is apparently an invention of
medieval Muslims who were unable to reconcile (1) the good things the Qur’an says about Jewish and Christian scriptures with both (2) their faith in Muhammad’s other “revelations” and (3) what Jewish and Christian scriptures actually say. In order to save (2), they came to the conclusion that they had to reject (3).