This verse is the "Sovereign Audit" of the Quran. It is perhaps the most uncomfortable verse for any Muslim apologist because it explicitly subordinates Muhammad’s revelation to the testimony of the Jews and Christians who preceded him.
Surah 10:94:
So if you are in doubt, about that which We have revealed to you, then ask those who have been reading the Scripture before you. The truth has certainly come to you from your Lord, so never be among the doubters.
If I tell you, "If you aren't sure about this math problem, go ask the math teacher," I am certifying that the math teacher knows the truth.
Allah tells Muhammad that the way to resolve doubt about the Quran is to check with those who read the Bible.
If the Bible-readers say, "Wait, our book says God has a Son and he died on a cross," and the Quran says, "God has no son and he didn't die," then the very source Allah sent Muhammad to for verification has just invalidated the Quran.
Most modern Muslims claim the Bible was already corrupted by the 7th century.
If the Bible was corrupted, why would Allah tell Muhammad to go to a corrupted book to find "The Truth"? That would be like a teacher telling a student, "If you're confused, go look at that book of lies and errors over there."
By directing Muhammad to the People of the Book, Allah is giving a divine "Seal of Approval" to the Bible manuscripts they were reading in the 7th century.
The verse ends with "Never be among the doubters."
The only way for Muhammad to move from "doubt" to "certainty" is through the testimony of the Bible.
If the Bible is the cure for doubt, the Bible must be the higher authority. If the higher authority (the Bible) contradicts the lower authority (the Quran), the lower authority must be rejected.
Was the Bible corrupted when Allah told Muhammad to ask the Bible-readers to resolve his doubt?
If it was corrupted, why did Allah send him to a corrupted source? If it wasn't corrupted, then why does that 'uncorrupted' Bible contradict the Quran?
The Quran has commanded its own followers to use the Bible as the final auditor of truth.