Ans.So what you're saying is we can't rely on what Jesus said and there is no record of what he said.
So, everything in the NT is a lie and the Quran was written 600 years after that.
The NT authors present it as the words of Jesus, how can we rely on anything in the NT?
Why ask me to answer about NT? You should ask the experts who have traced the history of each book of OT and NT.
There is no shortage of such books. Jesus spoke Aramaic language, which is a dialect of Hebrew language spoken during the time of Jesus.
But each and every book of NT is written in Greek. So it is very clear that "Not a single word which Jesus spoke is preserved in any NT book" (except for a few Hebrew words which we find in Gospels).
All of Jesus' chosen Apostles were Jews and they also spoke Aramaic or Hebrew. This shows that NONE of Gospels were written by Chosen Disciples of Jesus.
Who actually wrote them , and who translated these books into Greek language is not known.
I do not say that every translation is 100 % wrong or 100 % right. But you loose the originality, because now it is the based on understanding and capability of the translator, as to how good or how bad he did the job.
That is which makes these NT books unreliable (contradictions, mistakes, errors and interpolations are other reasons for unreliability)
Now coming to your observation that how come Quran a book "written" 600 years after NT books could tell the "Truth" about Jesus?
Well, you should know the Source of Quran. If it is revelation from God Almighty (as it claims), then time in immaterial , because God has full knowledge about all events past, present of future.
The same God, which told Moses about events that happened thousands of years before Moses (creation of heavens and earth, stories about Adam, Noah, Abraham, Ishmael, Isaac and Jacob, Joseph etc).
How come Moses knew about these events, which happened many centuries before him?
And why the same God cannot tell the truth to our prophet who came 600 years after jesus?
So really there is no force in your arguments.