That's really ahistoric Paul did not wipe out the early Christians. If he did anything it was spread Christianity so much in Rome and Greece that it somewhat lost touch with its Jewish origins but Paul was anything but an antisemite having himself come from a Jewish background and frequently preaching against any form of racial hatred

Also the Mithras stuff is an urban legend and the similarities pretty much begin and end with rituals involving wine and being popular in the Roman army and Christianity didn't catch on in the Roman army until after the relevant time period either. Additionally it is bizarre to conclude that Saul a Jewish religious authority would have suddenly decided to go from persecuting what he viewed as a heretical Jewish sect and instead convert it into a cult of a god popular in an army that he wasn't in and was occupying his country. Also the early Christians weren't wiped out by Saul or even nearly so. The early disciples of Jesus like Peter were still alive and didn't take issue with Paul's teaching either