If we look at the author's own descriptions of their work, it is clear that they did not intend their readers to see the account as fictional. All four Gospels have a straight-forward, narrative style which contains none of the flourish and fanciful writing of myths from that era. Both John and Luke expressly state in their Gospels that they are writing accounts of actual events. Luke's Gospel begins, 

Many people have set out to write accounts about the events that have been fulfilled among us. They used the eyewitness reports circulating among us from the early disciples. Having carefully investigated everything from the beginning, I also have decided to write a careful account for you, most honorable Theophilus, so you can be certain of the truth of everything you were taught.

Similarly, John concludes his Gospel by writing, 

This disciple is the one who testifies to these events and has recorded them here. And we know that his account of these things is accurate

Finally, in describing the stories and testimony that they have been preaching through the land, Peter writes, 

We did not follow cleverly invented stories when we told you about the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty. For we were not making up clever stories when we told you about the powerful coming of our Lord Jesus Christ. We saw his majestic splendor with our own eyes. (2 Peter 1:16)

There are also important clues in the Gospels which suggest that these are not fables. To begin with, the Gospels are not very flattering of the disciples. There are several passages which portray the disciples as dimwitted (Matthew 16:23) or petty and selfish (Mark 9:34). Consistently, the Gospel highlights the disciples' lack of faith, such as when Jesus calms the storm or before he feeds the 5,000. Peter comes off particularly bad in the Gospel accounts for his betrayal of Jesus the night of his arrest. There would be no reason to add these accounts to a myth or fable, and no author would intentionally add anecdotes which make them look bad.

More convincing are the accounts which expose Jesus' limitations, such as his inability to perform miracles in his hometown or the fact that he does not know the hour of his return. Tales such as these which limit the power of the hero are very unlikely items to include in a myth.

If the stories were mythical, it seems unlikely they would have included such difficult teachings. Some of what Jesus taught is very strict and demanding. Jesus did not teach us to "try hard" or "do our best"; He taught us to be perfect. He taught us to love our enemies, to forgive all offenses and to follow him at all costs. A fictional tale about a fictional character would likely have included less demanding precepts.

We need to acknowledge that there is no evidence of anyone coming forward in the first century to dispute the claims of the apostles. There are accounts of Jesus in ancient Jewish and Roman texts, and while they claim that Jesus' power came from sorcery, these sources acknowledge His life, death and even the miracles He performed. If Jesus were a fictional character, these accounts would dispute the apostle's tale, or more likely, would not exist at all.

Finally and most importantly, we have to acknowledge that the apostles continued to preach the Gospel for the rest of their lives even though it meant torture, imprisonment and death. The idea that these accounts were fictional is totally inconsistent with the way the apostles were willing to sacrifice their lives. No one would be willing to die for a story they made up. We are left with the inescapable conclusion that theses stories were not intended to be fictional. We might conclude that the disciples were mistaken for one reason or another or we can conclude that Jesus lived and was resurrected just as they said, however the facts clearly point to four men who intended to tell the truth.