In 1 Corinthians 11:17-34, Paul addresses issues related to the communal means the Christians in Corinth were practicing. In the process, in 1 Corinthians 11:23-26, Paul cites the following famous material:
For I received from the Lord what I also passed on to you: The Lord Jesus, on the night he was betrayed, took bread, and when he had given thanks, he broke it and said, “This is my body, which is for you; do this in remembrance of me.” In the same way, after supper he took the cup, saying, “This cup is the new covenant in my blood; do this, whenever you drink it, in remembrance of me.” For whenever you eat this bread and drink this cup, you proclaim the Lord’s death until he comes.
Paul explicitly says this is tradition that he had passed on to the Corinthians in the past. And we have no choice but to accept this claim, since there are too many unanswered questions in the material for it to represent new teaching that Paul was passing off as previously-presented material. The passage raises but does not answer numerous questions: “Betrayed” (or “handed over”) by whom? To whom was Jesus speaking? What night, when and where?
Mainstream historical critical scholarship concludes that there is a connection between this tradition, passed on orally by Paul to the Corinthians and then mentioned again in a letter probably written in the mid 50s CE, and the material that is clearly a version of the same tradition found in Mark 14. Most scholars date Mark’s Gospel around a decade or perhaps two later than 1 Corinthians.
What mythicism does with 1 Corinthians 11 is, on the one hand, refuse to allow the slightly later Gospel of Mark to shed light on it, while on the other hand, posits that Paul is referring to a heavenly occurrence in a mythical realm. For the latter understanding of the passage there is no explicit evidence in Paul’s letters, much less earlier. Mythicism then has to posit as well that someone in the years between 1 Corinthians and Mark’s Gospel reinterpreted the mythical celestial story as an earthly one – a process of reinterpretation for which, once again, we have no evidence, including there being nothing in the Gospel of Mark that indicates that it is engaging in such a radical resetting of the traditional material it incorporates.
Historians and scholars always have to fill in gaps in our piecemeal evidence from antiquity. But convincing mainstream scholarship tries to limit the number of ad hoc assumptions brought in and to offer not merely a possible interpretation of the data, but one that is relatively probable and does as much justice to as much of the relevant evidence as possible.
Can anyone seriously claim that there is a mythicist scenario that offers that? I think the example of 1 Corinthians 11 illustrates well why the vast majority of scholars find mythicism unpersuasive. It takes evidence that can be explained in a relatively straightforward way, and posits instead a scenario for which there is no evidence and claims that it is preferable.
I think I understand why this is done. It is much simpler to say “Jesus didn’t exist” than to say “Jesus almost certainly existed, and he probably said X, but may not have said Y and almost certainly didn’t say Z.”
But making the lives of contemporary people easier is not a valid criterion for drawing historical conclusions.