I notice in a Qur'anic commentary [ Surah 4:153-176,AYA ] stating that 'the people of the Book went wrong: The Jews in breaking their covenant and slandering Mary and Jesus....and the Christians in raising Jesus the Apostle [Hebrews 3:1] to equality with God' by means of the Trinity doctrine.
Note: by those 'Christian' in name only, not a first-century Christian teaching. The trinity doctrine of non-biblical origin was introduced by those who had fallen away from first-century teachings of Christ after Jesus and his apostles died.- Acts 20:29,30.
Why is the Quran viewed as authoritative concerning events that were recorded long before the religion even existed. In no category used to judge the reliability of historical claims apart from faith is the Quran more reliable or authoritative on Biblical matters. In fact while the Bible implies that Muhammad was not a prophet the Quran instructs people to judge his words by the Bible and affirms the Torah as well.
Sahih International
And let the People of the Gospel judge by what Allah has revealed therein. And whoever does not judge by what Allah has revealed - then it is those who are the defiantly disobedient.
Surat Al-Ma'idah [5:47] - The Noble Qur'an - ?????? ??????
There are only two possabilities here.
1. Muhammad screwed up and said to judge his book by the Bible because his unfamiliarity with it (demonstrated in many places) made him believe that he was consistent with it and in reality it does anything but affirm the Quran.
2. It meant to only use the parts that were pure and uncorrupted.
I think 1 is infinately more likely. Why in the world would anyone especially a prophet tell someone to use a corrupt text to judge his book by? If that was the case how in the world would poor mostly illiterate folks know what part is pure and what isn't (1500 years later and with computers the issue is still hotly contested). If the text you said to use to judge the Quran condems the Quran instead by it's being corrupted would not an actual prophet give strict details about what is and what is not corrupted? If so how can I judge a book by the one that it corrects. That makes no sense. It is like saying Isaac Newton's work was perfect and validates mine except where I say it does not.
The first possability is consistent with what is known about the Quran and Muhammad. The second is illogical, self serving, and inconsistent with what is known.