Was Mark's Gospel a Work of Fiction?

As I've argued before, history is a slender reed to hang a religious faith on. The evidence for an event happening in the past is always going to be less than what we personally experience--always! We know all to well about forgeries, faked evidence, and fraudulent claims to take anything at face value, especially when it comes to the past, and exponentially when it comes to some miraculous story in the ancient superstitious world.

German critic Gotthold Lessing (1729-1781) challenged us in this regard:

We all believe that an Alexander lived who in a short time conquered almost all Asia....But it might still be possible that the story was founded on a mere poem of Choerilus just as the ten year siege of Troy depends on no better authority than Homer’s poetry. Link.

So why is it not the case that Mark's Gospel, the first one, was written as a work of fiction in the hopes of starting a religious movement of believers, or in providing a fictional story for an existing group of believers? I cannot discount this possibility, just as Lessing could not discount the possibility that Alexander did not conquer almost all Asia.