Is the Torah the word of HaShem?
Who says so? The collective memory of the Jewish people, the revelation at Sinai. 600,000 Souls were witness's to HaShem and his words and Laws. Our ancestors job was to pass down the unaltered words and Laws.
Who spread the word of Christianity? Paul
Who was a witness to any of the miracles Paul described? Christians believe Paul said many things.
But was what he said true? Who can attest?
Why should people believe Paul?
Who spread the Muslim's word of their Prophet?
That theirs is a book called the Koran is true, but the "stories" it contains, were true? Who can attest?
Why should anyone believe the Koran?
With Judaism it is different.
We deal with a collective memory passed down through the generations in something so simple as the Passover story.
Do you trust your Father and Grandfather....etc.
Why is it hard to believe that the Oral Law and Torah could not have preserved intact. Our forefathers knew that not one letter could be altered!
We have the Covenant ...
Only the Jewish people have the Covenant!