Who corrupted the scriptures?, Why did they corrupt the scriptures?, When did they corrupt the scriptures?, What is your historical proof that this occurred?
Who corrupted the scriptures?
People.

Seriously, Muslims don’t seem to have a definitive answer to this question. Some say that Paul corrupted the message of Jesus, yet others say that it was through the translations that the scripture was corrupted.
Why did they corrupt the scriptures?
Again, there is no definitive answer. Moderate Muslims would usually say that corruptions were inserted by people who did not realize their mistakes or by individuals who meant well, but were ultimately wrong. More conservative and strict Muslims would say that these scriptures were purposefully distorted by those seeking political and worldly power.
When did they corrupt the scriptures?
Same story, no single answer. It has been said that during the Babylonian captivity, certain Hebrew scriptures were destroyed and re-written by memory. During later fiascos, other manuscripts were lost and redone. It is certainly unfair to expect that the early Christians realize that Jesus had not died given the historical arguments to the contrary. How would they have known? Some Muslims would say that certain apocraphal books such as the Gospel of Barnabas (written around 150 AD), are the more accurate stories of Christ’s life with stories of childhood miracles like the clay pigeon and his fake death.
Historical proof?
There really is not a whole lot aside from a few spurious writings like Barnabas. It is mostly the Islamic presupposition that Christ did not die which controls this belief. In reality, the early Church rejected such notions as “Gnostic” due to the known truth of Jesus’ passion and resurrection. Most of the Muslim teaching here rests not on whether Christ died or not, but rather on the belief that if the Qur’an gets so many other things right, then this must be right, too.
Prayers and petitions,
Alexius
