The idea of being post christian assumes the institutionalized form that the Christian church took after being adopted as the official cult of the Roman empire can properly be considered to be Christian. My working assumption (along with the early Unitarians and the rest of the radical wing of the Reformation) is that institutionalized mainstream Christianity with its worship of domination and violence constitutes a departure from the gospel of Jesus.
Post christendom? I claim that stance enthusiastically.
I am with Tolstoy, Christianity is not a failure, it has never been tried. (Except for among dissenting intentional communities.)
You have a good point that the orthodoxy that arose in the first centuries after Jesus's death did not really originate with Jesus or necessarily with the first generation of his followers, so in that sense, "post-Christian" could be said to be a misnomer.
But in another sense, I think that we have a modern sensibility that is different than what Jesus's generation had, and necessarily a post-Christian theology isn't going to be the same as what any early Christians were likely to have. For one thing, I doubt (and I woudl stand corrected if I am wrong) that most early Jesus-followers were religious pluralists, and I think that a modern post-Christian theology must necessarily incorporate religious pluralism into its theology. I also think that most people of Jesus's generation (again, I could be wrong) were believers in divine omnipotence and divine intervention, whereas a post-Christian theology recognizes that tales of divine intervention in human history are myths not to be taken literally. Our world view necessarily incorporates a scientific, modern understanding of nature that wasn't part of the early Christian understanding of the world.