The testimony of two witnesses is all it takes. There were many more...
Because as many of us know, the Koran----Mohammed claims the angel Gabriel was sent by Allah to reveal the Koran to Mohammed---has many references ro Jesus and says that Jesus was a great Prophet.
End of time: Mohammed even says in the Hadith---collection of his sayings and teachings---that Jesus will come at the end of time to judge man and break all the crosses, a reference to the cross of Christianity.
Also, the sacred Koran has a chapter---surah---dedicated to the Virgin Mary herself. But if there is no historical Jesus, then the chapter dedicated to Mary is in error.
And would millions of Muslims dare say that the Koran is in error?
I don't think so.