There are at least three completely plausible possibilities.
1. These people really believed in the magic and miracles they thought they had witnessed and were simply mistaken. Plenty of crazy people seriously believe in their delusions. I'm sure that some of the oracles of Delphi really thought that they were experiencing visions from the gods. It is not unreasonable to think that these people were sincere. Sincerity has no bearing on veracity, however. As above, plenty of people genuinely believe things that aren't true. Any religious figures could have been delusional. We don't know.
2. They were charlatans. Lots of people make up stories for fame, glory, or money. Joseph Smith, for example, was a known charlatan before creating Mormonism. It is not unreasonable at all to suspect that he retained those motives in creating a new religion. Paul or Jesus or anyone else living two thousand years ago could certainly have had the same motivations. Any religious figure, recent or in antiquity, could have done this.
3. The stories were edited later. What was originally the fairly mundane life story of a Jewish preacher and his friends could have been embellished by future retellings to incorporate supernatural elements ("miracles") or parts of other legends. Or they were simply mistold in future iterations. Urban legends still exist and are modified as they are retold even in this modern age of instantaneous communication and the internet. In the middle of a desert in the iron age, it would be easy to make these mistakes even with the best intentions. With mistakes and retellings, the figures in question may never even have existed at all.
All of these possibilities can and do happen all the time. The spread of a particular story has no bearing on whether or not it is true. Plenty of people today believe in alien abductions or Elvis still being alive. Now, we have no more evidence of the motivations of the authors of the New Testament at all. Nor do we have evidence of the motivations of the authors of any legends. Not those of Theseus and the Minotaur, of the Oorochi, of Gilgamesh, or any others. But we reasonably attribute their stories to one of the above three possibilities. A lot of people passionately care about the New Testament, or about the Quran, or about the Book of Mormon, but passionate caring doesn't mean that we except a story from scrutiny. All six of those stories are likely fiction for exactly the same reasons. Lots of people throughout history have fought, killed, and died for their particular story. We reasonably conclude that all of their stories were wrong.