We're not so different from the people at the time of Columbus that were taught the world was flat. They didn't (most couldn't verify it), but everyone 'knew' it was true. That had been taught it, they had books describing it, and maps showing it. They had as much evidence as I do about mars.
Many people really have seen mars through telescopes, and a number of rocket scientists have built devices to go and look at it indirectly. THOSE people KNOW mars exists. The rest of us believe, blindly, what we are told about it. I don't doubt mars exists in the least, only because I _choose_ to believe it exists.