Those who believe in the Jesus myth theory (JM) point out that there are many similarities between things in the Gospels and in pagan myths. I would say that those similarities are far fewer than they claim. It is also a problem that they don’t compare the Gospels with one pagan text but with a phrase here and sentence there and a sculpture somewhere else. All those things are serious problems for the JM. But there is another major problem.
Imagine that an inscription dated to 50 BC was found in North America or Australia (put aside limitations of oral cultures, this is a thought experiment). In this inscription, in one story we we find an account of the great spirit becoming a man, teaching truth, being betrayed by a friend, being executed but eventually rising from the dead. That sounds very close to the story of Jesus. In fact we have nothing this similar among existing pagan myths. What would be the significance of this text? Could we conclude that the Gospel writers copied the story that was found in North America or Australia? No matter how similar the stories may be, unless the Gospel writers had access to the story, there is no way that the one story was responsible for the other. There is a difference between similarity and causation.
Of course, JM focuses on Greco-Roman and Egyptian myths and they would argue that the Gospel writers had access to them. They likely would have had some knowledge of some of the other religions in the area. But the JM does not argue that the Gospels were based on one local form of a religion. They take all of the versions of religions of that area over a long period of time (some buried in a pyramid for a thousand years and some written after the New Testament) and try to demonstrate from that there is evidence for the source of the story of Jesus.
If a person wants to argue that Jesus is based on pagan myths, they need to do more than demonstrate a similarity of a phrase or symbol. That person would have to demonstrate the likelihood of causation, through geographical, chronological and cultural proximity. Even then, there would still be many questions.
