I come back to this subject several times, and have been prompted so again. The question is, what makes a Christian.
It isn't just about someone who imagines they have "a relationship with Jesus" but rather an internal orientation towards Jesus about whom they have a set of beliefs. It is always, as Diarmaid MacCulloch has said, a 'cult of the individual', that is to say the focus of salvation.
Those of us who have looked at biblical criticism over time can come to a view like this: that Jesus himself pointed away from himself and to God; that Jesus had a short mission and put himself in harm's way as a demonstration to this God of faithfulness that now was the time to bring in the transformation. He had prepared people for it by healing and teaching. So much of the passion narratives simply do not stand up, but it can be said he was picked up by the authorities and disposed of like so many at the edge of empire.
The titles for Jesus escalated in the early charismatic Christian community, in their expectation that he would be the transformed messiah, the messenger of God and starting gun for the new reality. Put into their core rituals, it nevertheless didn't happen, and one aspect became what was unique from Judaism - the notion of the second coming given the need for an Ascension. This has been picked up by Islam: Jesus also returns when Islam has its end moment.
Now to be a Christian, it seems to me, is to agree broadly with the interpretation of that community: that is, to believe in some reality of the resurrection, and then with the proto-orthodox and later orthodox, to believe in Jesus as a particular incarnation of God, that is the titles as escalated that ensure Jesus's primacy.
This has to be doctrinal and communal. Historiography and certainly science does not allow these beliefs to have any hold other than what a community believes. Some postmodernists (conserving postliberals) hold that these beliefs can be held as a kind of badge of identity and 'performance' - but this is something of a get out clause from contemporary reality.
Others just pick Jesus as a kind of familiar hero, someone good enough to follow, and yet believe in a general incarnation. They probably don't believe in a resurrection and, if they do, they ought to examine their beliefs. Some do and are Arians, and Arians are definitely Christians as Jesus is simply subordinate to the Father, who as Christ was produced to create the universe.
To me that's mythological drivel, and I neither believe in any actual resurrection (that involves a single consciousness - peoples' subjective spiritual experiences are all over the place, retold and unreliable) nor in a particular incarnation. Jesus was a mistaken end-time rabbi with interesting ethical reversals and means to increase love and compassion.
Thus I am not a Christian and I'm quite happy about that.