Gerd Ludemann believes that people cannot be Christians if they don’t believe Jesus was raised from the dead.
“If we take seriously the nature of historical knowledge and our own human dignity, we cannot be Christians any longer. Since Jesus did not rise from the dead, those who nonetheless continue to claim that title are deceiving themselves.” (The Resurrection of Christ, p 205)
This conclusion trails Ludemann’s attempt to demolish various “vain” attempts to remain confessional (the first four deny a literal resurrection, the last two accept it):
the “vain” kerygma approach of Bultmann, which maintains that the proper object of Christian faith is the proclamation of Christ, regardless of the historicity of that proclamation (pp 193-195)
the “vain” objective vision approach of Grass, which maintains the reality of visions whether or not they can fit into a scientific worldview (pp 195-197)
the “vain” metaphorical approach of Kessler, which insists the resurrection was real but non-literal and metaphorical (pp 197-198)
the “vain” replacement of the risen Christ with the historical Jesus, the approach of many liberal scholars today (pp 198-199)
the “vain” theological approach of Wright (pp 199-202)
the “vain” fundamentalist approach of Hartlich and Broer (pp 202-203)
I don’t know that any of these approaches are inherently “vain”, even if I personally eschew them as a Unitarian. Christianity is an evolving religion like any other, and I find it curious that a literal understanding of the resurrection would be viewed as a prerequisite for being Christian by an atheist historian.