Mark’s Gospel is the founding document of the united Christian church; it was classified as canonical
from the very start. In addition, the leaders of the three individual churches, James, John and Peter, are presented jointly in Mark’s Gospel as witnesses to the core statements of faith of all three single churches.
The writer of Mark’s Gospel adopted stories about Jesus handed down in the Jesus Groups
led by James, John and Peter. Cf. in detail Johannes Neumann, War Markus ein Dichter? in: Neumann.: War Jesus Statthalter von Galiläa?, p. 43-92, here p. 51-62.
The evangelist adopted stories about Jesus from Galilee that originated as oral traditions handed down in the individual churches. The Jesus stories from Jerusalem are based on interpretations of Jesus’ death by the individual churches and their cultic application.