There are a number of different versions of the Jesus myth theory. However, for any of them to work, they need to deal with Paul. Many mythicists claim that Paul never speaks of the earthly historical Jesus but rather always speaks of a glorified Christ. The thought is that Paul is using Jesus Christ as a mythical symbol for spiritual truth and is not considering him to be a historical figure.
There are many things that I can say about that. However, there is a passage in Romans that is impossible to harmonize with the Jesus myth theory.
“They are Israelites, and to them belong the adoption, the glory, the covenants, the giving of the law, the worship, and the promises. To them belong the patriarchs, and from their race, according to the flesh, is the Christ who is God over all, blessed forever. Amen.” (Romans 9:4–5 ESV)
Here Paul is talking about the advantages of the Jewish people. There are many great things about Israel based on what God has done through them. As a person raised as a Jew, Paul would have understood all of these things as historical. Notice that one of the things that Paul mentions is that Jesus according to the flesh came from the Israelites. I cannot think of any way that this could be true if the Jesus myth is true. If Jesus is just a symbol or a myth, he is not connected “according to the flesh” to Israel any more than any other nation. The only rational way to understand this passage is to see Jesus as being physically related to the Jews as a historical figure.