To make the point more clear.
Just because Allah allowed humans to change the message which was taken to be as His word on Earth, doesn't mean that Allah was powerless to stop that.
It is the same as like when God allows all the suffering in the world, it doesn't mean that He is powerless to end it. It is according to His plan for humans.
The truthfulness of whether a particular alleged God is the actual God, depends on testing the promises that He makes. As far as I know, Allah never promised that all the divinely inspired books, in the hands of humans, will remain corruption-free till the end of time. Therefore, if such happens, it cannot be used as an argument for His refutation. This is according to, what is apparently, His plan for the humans.
I guess that is the difference between the Bible and the Quran, in this context.
The doctrine of preservation in regard to the Bible means that the Lord has kept His Word intact as to its original meaning. Preservation simply means that we can trust the Scriptures because God has sovereignly overseen the process of transmission over the centuries.
At the same time, Christians are aware that we do not possess the original writings/autographs. What we do have are
thousands of manuscripts from which the original writings can be ascertained. By thorough examination and comparison of those manuscripts, it is determined what the original writings stated. This does not mean that there are absolutely no differences between the manuscripts. But the differences are extremely small and insignificant and do not in any way affect the basic teachings or meaning of God’s Word. The differences are things like minor spelling variations. We should keep in mind that this would not and does not affect the accuracy of Scripture, nor does it mean that God has not preserved His Word. God has supernaturally kept or preserved His Word.
The early scribes, whose jobs were to make exact copies of Scripture, were very meticulous. One example of their scrupulous precision is the practice of counting all the letters in a given book and noting the middle letter of the book. They would then do the same for the copy to make sure it matched. They employed such time-consuming and painstaking methods to ensure accuracy.
Further, we can take note of the following verses that demonstrate God’s plan to preserve His Word. In Matthew 5:18, Jesus said, “I tell you the truth, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished.” In this verse Jesus declared that not even the smallest stroke of a letter would pass away until all is accomplished. He couldn’t make that promise unless He was sure that God would preserve His Word. Jesus also said, “Heaven and earth will pass away, but my words will never pass away” (Matthew 24:35; Mark 13:31; Luke 21:33). Jesus again affirms that God’s Word will not pass away. God’s Word will remain and accomplish that which God has planned.
The prophet Isaiah, through the power of the Holy Spirit, stated that God’s Word would remain forever. “The grass withers and the flowers fall, but the word of our God stands forever” (Isaiah 40:8). This was reaffirmed in the New Testament when Peter quoted the same passage and referred to it as “the word that was preached to you” (1 Peter 1:24-25). Neither Isaiah nor Peter could make such statements without the understanding of God’s preservation of Scripture.
In Psalms 12:6-7 it says, "The words of the LORD are pure words: as silver tried in a furnace of earth, purified seven times. Thou shalt keep them, O LORD, thou shalt preserve them from this generation for ever."
We have these promises from God in the Bible, therefore, we trust that all that is written in the Bible as God wanted all of it to be there.
Thanks for the conversation!