It's based on the Revelation at Sinai. No eyewitness is necessary to believe G-d always speaks Truth, "historical facts" are often denied, and whether or not a prophecy has been "fulfilled" is often a matter of personal opinion, prejudice, or logical fallacies such as affirmation of the consequent.
Sinai proves everything.
Not necessary, true.
Yet Jesus Christ did have His life, teachings, words and deeds historically documented just the same through the prophets, the Apostles, Disciples, and many other eyewitnesses.
Btw -- can you point to a single Biblical prophecy that has been proven wrong?