Gerd Theissen defends the existence of a Passion source in chapter 4 “A Major Narrative Unit (the Passion Story) and the Jerusalem Community in the Years 40-50 CE” in The Gospels in Context (London & New York: T&T Clark, 1992), 166-199. While Theissen dates this source between 40 to 50 CE, he dates the finished Gospel of Mark to after 70 CE. His major arguments are that there was a common source underlying the Markan and Johannine Passion narratives which were independent of each other and the writer of the source was directly familiar with some of the characters named therein.
Starting with the disagreement between the Markan and Johannine chronologies on the precise date of Jesus’s death, Theissen believes that Mark’s source actually agreed with John that Jesus died on the day of preparation before Passover. This is the real reason for why Jesus was hastily tried at night for there could be no judicial proceedings on the Passover (Mark 14:1-2), why Simon of Cyrene came from the field despite work being forbidden during the Passover (15:21), and why Mark 15:42/John 19:42 have the “day of preparation” yet Mark inserts a relative clause to link it with preparation for the Sabbath (166-168). Theissen is reluctant to state the exact length of the Passion source, though the correspondences between Mark 14:1 and John 11:43-47 onward may signal that it started here, but he is open to the source being shorter (Bultmann) or longer (Pesch) (168-169).
Second, Theissen builds on Pesch’s case about the “indications of familiarity.” Not naming the high priest may not demand that he was the currently in charge (contra Pesch), for the book of Exodus does not name the Pharaoh, but dropping Caiaphas’s name while his family was in power between 30 and 70 CE was risky. In contrast, Pilate could be named because it was easier to blame a former office-holder who was disposed of in 37 CE than the Roman office itself and Mark was more embittered against the priestly leadership. The naming of the sons of Simon of Cyrene indicates that they were well-known members of the community (cf. Acts 6:9). Mark 15:41 is not entirely clear on the familial relationship of Mary to James and Joses, so they too may have been well-known. This James was identified as “the younger/less” to distinguish him from James, the son of Zebedee, so the Passion source preceded the latter person’s death (ca. 44 CE). Characters were named by their places of origin – Nazareth, Magdala, or Arimathea – and these locations would mean little to those outside Palestine. While Matthew 27:16 calls Barabbas a notorious prisoner and Luke 23:19 is explicit about his crimes, Mark 15:7 assumes the audience’s knowledge of him by describing him “with” the rebels in the insurrection, thus leaving his level of involvement in their activities apart from his conviction for murder unclear. Last, the bystander who cuts the ear of the priest’s slave (14:47) and the young man who resisted arrest by fleeing naked (14:51-52) are not named; Theissen invokes the theory of protective anonymity to protect those still alive who ran afoul of the authorities.