This verse acts as a definitive statement on the sufficiency and perfection of the Mosaic revelation, characterizing the Torah not as a preliminary or "broken" text, but as a finalized, detailed, and merciful guide for its people.
Surah 6:154:
Then We gave Moses the Scripture, making complete upon the one who did good and as a detailed explanation of all things and a guidance and mercy that perhaps in the meeting with their Lord they would believe.
The Quran identifies the Torah as a detailed explanation of all things.
If the Torah was detailed and explained "all things" (laws, prophecies, nature of God), it provides a robust defense against later claims of textual alteration.
You cannot easily "corrupt" a book that is granularly detailed without it being obvious to the community. If it was the "detailed explanation" during the time of the Quranic revelation, it must be the same detailed text we have in our hands today.
Allah says he gave the Book to Moses to "complete" the favor.
If the Bible was corrupted, then the "completion" was a failure. If the favor was "completed" upon the one who did good, then that "good-doer" had access to the full, unadulterated truth.
By validating the Torah as a "complete" favor, the Quran inadvertently validates the historical reliability of the Hebrew Bible.
The verse ends by saying the Torah was given so that they "would believe" in the meeting with their Lord.
If the Torah's purpose was to lead people to a true belief in the afterlife and their Lord, it must have contained the Truth.
If the "Detailed Explanation" taught the Jews about the Covenant, the animal sacrifices, and the coming Messiah (as the Torah does), then those teachings are the "Guidance and Mercy" Allah is praising.
If the Quran then comes and changes those specific "detailed" laws, it is contradicting what it previously labeled as "complete."
Surah 6:154 is a high-water mark for the status of the Torah in the Quran. By labeling it a "complete favor" and a "detailed explanation of all things," the text effectively "seals" the Torah as a perfect divine work.
If the Torah is a detailed, complete, and merciful explanation of God's will, its testimony cannot be brushed aside by later claims of corruption.
If the "Detailed Explanation" says one thing and the Quran says another, the Quran is at odds with a book it has already defined as "Complete."