The fascinating phenomenon of the "cargo cults" that emerged on various South Pacific islands during World War II provides us with a modern example of how religions can grow. According to the reports, when American military planes began arriving with cargo during the war effort against the Japanese, the natives thought the airplanes were from the gods and that the cargo they were carrying were sent from their dead ancestors and intended for them.
But the white men, with their runways and their air traffic control towers, lured the airplanes away from the natives, and thus were taking what belonged to them. So in an effort to lure the planes to their rightful beneficiaries, they built mock runways and air towers hoping that a plane would descend from the heavens and bring them cargo and make them all rich and prosperous. They emulated military routines: marching, formations, saluting, some even would write "USA" on their chests—all to do what they thought the white man was doing to attract the attention of the planes.
Then there is the legend of John Frum, parralleling the story of Jesus. John Frum is said to have been a man who began appearing to the natives wearing Western clothes and promising them material riches. His description varies, from being short to tall, and from being black to white. No one knows if he actually existed, or who he was based on. The most reasonable guess is that someone mistook "John from America" and made it into a name.
This phenomenon illustrates how religions can quickly spring up and how gullible, uneducated peoples can fabricate the existence of legendary figures, whether they're based on real people or not. And this all happened within living memory. Now given our current knowledge of science and human nature, given the natural psychological and evolutionary tendencies we have to make up and believe fantastic things and to attribute agency and intentionality to nature, and given the notorious unreliability of eyewitness testimony, the John Frum phenomenon is good evidence why the probability of the New Testament being a reliable historical document is extremely low.
Watch this video explaining the cargo cults: