This verse defines the very truth-status of the Quran based on its relationship to the Bible.
Surah 2:91:
"And when it is said unto them: Believe in that which Allah hath revealed, they say: We believe in that which was revealed unto us. And they disbelieve in that which cometh after it, though it is the truth confirming that which is with them. Say (unto them, O Muhammad): Why then slew ye the prophets of Allah of old, if ye are (indeed) believers?"
The verse does not say the Quran is the truth despite the previous book; it says it is the truth because it confirms what they already have.
If the Quran is the "Truth" (Al-Haqq), and its truth is tied to "confirming what is with them," then the book "with them" must be the absolute Truth as well.
You cannot have a "True" book that confirms a "False/Corrupted" book. If the 7th-century Bible was already corrupted (containing the "lie" of the Trinity or the "falsehood" of the Crucifixion), then a Quran that "confirms" it would be confirming a lie—meaning the Quran could not be Al-Haqq.
In this verse, the Torah is the independent variable and the Quran is the dependent variable.
The Jews are told to believe in the Quran because it matches what they already have. This makes the Torah the standard and the Quran the thing being tested.
If the standard (the Torah) is broken, the test is meaningless. If the standard is intact, the Quran fails the test because it doesn't actually match the Torah's theology (as we discussed with the sacrifice of Isaac vs. Ishmael).
The end of the verse shifts to a moral attack: "Why then slew ye the prophets...?"
This is a classic "red herring" in the text. When the Jews point out that they only believe in what was revealed to them (their own Bible), the Quran doesn't argue that their Bible is corrupted. Instead, it attacks their ancestors' behavior.
By attacking their behavior rather than their book, the Quran implicitly admits that the book itself is fine. If the book were the problem, the rebuttal would have been: "Why do you believe that book? It's been changed!" Instead, it says, "Why didn't you follow the prophets your book tells you about?"
Surah 2:91 says the Quran is 'The Truth' specifically because it confirms what was 'with' the Jews in the 7th century.
In which scenario is the Quran actually true? It seems the only way the Quran can be 'The Truth' is if the Bible it confirms is also the Truth. And if the Bible is the Truth, Islam is unnecessary.
"This verse essentially forces the Muslim to defend the integrity of the 7th-century Bible in order to prove the Quran is "The Truth," which then leads them directly back into the heart of the Islamic Dilemma.