There are a number of reasons. One is that the entire Jewish nation experienced God communicating with them at Mount Sinai. But I think there is a more basic reason, one that preceded the “Sinai Experience”, and shows the existence of a Creator who interacts with the Creation.
Perhaps the following story will help explain this idea. Sir Isaac Newton, the famous physicist, had a colleague who was an atheist. Newton frequently argued theology with this colleague.
One day when the atheist came to visit Newton his eyes fell upon a most beautiful sight. Sitting on Newton’s desk was an exquisite brass machine that depicted the solar system in three dimensions.
"How beautiful!" remarked the atheist.
"You haven't seen anything yet!" said Newton. "Do you see the small lever on the base? Move it towards you."
As the atheist moved the lever, the entire engine slowly came to life. At its center the orb of the sun started to revolve. Further out, turning on brass cogs, the earth and the planets began their revolutions around the sun; each planet accompanied by its own moons, all moving in wonderful precision.
"This is amazing!" remarked the atheist, "Who made it?"
"No one" replied Newton, deadpan.
"What do you mean no one?"
"No one. It just sort of fell together, you know"
"No I don't know! I insist you tell me who the maker of this priceless object is. I refuse to believe that this object merely fell together."
"This..." said Newton, pointing to the astrolabe, "this you insist has to have a maker. But all this..." Newton spread his arms wide, indicating the Creation, "how infinitely more beautiful and complex, all this you insist has no Maker?"
You don't have to be able to invent the First Law of Motion to read the world like a book.
Just as the book testifies to the existence of its writer, so too the world testifies to the existence of The Divine Author.