Yesterday, the 9th of Tevet, is a significant date on the Jewish calendar, but for reasons that are hard to pin down. It was ordained as a fast day during the Geonic period, but for reasons that are unclear. Some versions record that this date it the yahrzeit of Ezra ha-Sofer. Another tradition lists it as 'unknown' (see SA oc 580:2), but commentators note that it may be the yahrzeit of 'Shimon', an early Christian who was planted by the Sanhedrin in the nascent movement in order to divorce it from Judaism. This issue is the topic of an article by Professor Shnayer Leiman in JQR 1983. It relies heavily on the medieval Jewish work 'Toldot Yeshu', which identifies this Shimon with St. Paul.
I've always found this particular tradition, that Pauline Christianity was actually a Jewish invention designed to transform early Christianity into a non-Jewish religion, fascinating. The life of Paul - particularly his origins as a 'Pharisee' who studied under Rabban Gamliel and his 'transformation' on the road to Damascus - invite this type of theorizing.
There's a Gemara, much earlier than anything else that discusses this tradition, which I speculate might contain the kernel of this legend:
תלמוד בבלי מסכת נדה דף כד עמוד ב
תניא, אבא שאול אומר: קובר מתים הייתי, פעם אחת נפתחה מערה תחתי, ועמדתי בגלגל עינו של מת עד חוטמי כשחזרתי לאחורי אמרו עין של אבשלום היתה
It was taught: Abba Shaul said: "I was an undertaker. It once happened that a cave opened beneath me, and I stood in the eyeball of a corpse up to my nose. When I returned, they said that it was the eye of Avshalom."
This is one of those truly bizarre Gemaras. There's another, similar story on the same page which talks about the 3-mile long femur of the King of Bashan, Og. There, the teller is either Abba Shaul or Rabbi Yochanan, and the difference might be whether the two stories are a unit. If they are, then I have yet to find a good explanation, because I just don't know what to do with Og's femur. I think I have an idea about Avshalom's eyeball, though.
Avshalom, the son who usurped the father's crown and tried to kill the father, the long-haired scion of David who is killed for his usurpation, is an easy allegory for Christianity and/or Jesus. His enormous eye represents Christianity's desire to conquer all, to recast everything in its own image; the large eye is a panopticon, symbolic of a religion that wishes to be an empire. Christianity only became such after Paul. Paul's Hebrew name, according to the Christian Bible, was Saul, the same as Abba Shaul's. Finally, the idea of Abba Shaul being an 'undertaker' (lit. 'a burier of corpses') is an apt metaphor for the job of divorcing Early Christianity from the Jewish mainstream. He wanted to kill the religion which threatened Judaism by turning it into something non-Jewish.
So do I really think that the tanna Abba Shaul is none other than St. Paul, who some Jewish traditions regard as the Sanhedrin's emmissary? No. Do I think that the Gemara thought so? Not really. This is just a bit of speculation; like I said before, though, this topic fascinates me.