This verse establishes that the Scripture must be accepted in its entirety, creating an impossible situation for anyone who claims to follow the Quran while rejecting parts of the Bible.
This verse was originally a rebuke to the Jews of Medina who followed parts of the Torah (ransoming captives) while ignoring other parts (killing and expelling their own people). However, the logic applies universally to how one treats "The Scripture" (Al-Kitab).
Surah 2:85:
"...Believe ye in part of the Scripture and disbelieve ye in part thereof? And what is the reward of those who do so among you save ignominy in the life of the world, and on the Day of Resurrection they will be consigned to the most grievous doom. Allah is not unaware of what ye do."
Most modern Muslims argue that we only have "parts" of the original Torah and Gospel. Surah 2:85 makes this position dangerous.
If you claim the Bible is "partially corrupted," you are forced to "disbelieve in part" of the very book the Quran commands you to believe in (Surah 4:136).
By trying to save the Quran from the Bible's contradictions, the Muslim apologist falls into the "grievous doom" mentioned in 2:85 for refusing to accept the Scripture in its entirety.
The Quran claims to be a "Confirmation" (Musaddiq) of the previous Scripture.
If the Quran confirms the Gospel, and the Gospel says Jesus is the Son of God, but the Quran tells the Muslim to reject that "part," then the Quran itself is commanding the Muslim to do what 2:85 forbids.
The Quran essentially tells the reader: "Believe the Gospel (Surah 5:47), but disbelieve the part of the Gospel that says Jesus was crucified (Surah 4:157)." This is the literal definition of "believing in part and disbelieving in part."
The verse implies that the Kitab (Scripture) is a singular, authoritative unit.
The Quran acknowledges the Biblical Canon. Since the Jews and Christians of the 7th century had a defined canon (the Bible we have today), Surah 2:85 demands that this specific collection be accepted as a whole.
If one accepts the "whole," they must reject Islam (because the Bible contradicts it). If they reject "part," they are condemned by Surah 2:85.
In the 7th century, the Jews were 'reciting the Scripture' (2:44) and were warned not to 'disbelieve in part' of it (2:85). This means the 'part' they were disbelieving was present and readable."
If you follow the Quran's advice and believe the whole of the Scripture that was with the People of the Book, You must believe in the deity of Christ and His sacrifice. If you reject those parts, you are doing exactly what Surah 2:85 says will lead to 'ignominy' and 'grievous doom.' How do you resolve this?"
This verse effectively removes the "partial corruption" safety net, forcing the believer to either accept the whole Bible (which disproves Islam) or reject the Bible (which violates the Quran).