Nick of A Streetcar Named MacGyver asked me during an e-mail exchange: "I'm interested to hear what you think the bare minumum would be for a belief system to be thought of as Christian. I tend to think it would at least involve a belief in the divine nature of Jesus, SOME duality in the afterlife (otherwise, what is Jesus providing salvation FROM?), and an objective system of right and wrong (even if we can't entirely know it)."
It's a question that comes up a lot from students. And most assume similarly that the divinity of Jesus would be a dealbreaker.
But that's the revelation of research into the earliest generations of Christians. There were a bewildering variety of perspectives on this very basic point. The gospels themselves show a range. Among the synoptics, Mark has an adoptionist perspective (the baptism is the moment at which Jesus is anointed as a special messenger of God), while Luke and Matthew give him somewhat more developed superheroic characteristics (like a miraculous birth). All three agree, however, that Jesus is the Messiah -- a special human being given a divine mission to restore Israel to prominence. They give him the title "Son of God," which is the title of the Davidic line of kings, not an equivalence with God. Jesus calls himself "son of man" in the synoptics, which could mean simply human (the Hebrew word for man is 'adam, so he'd be saying "son of Adam") or could be a reference (as Matthew takes it to be, and has Jesus say in before the Sanhedrin) to Daniel's vision of "one like a son of man coming in glory." None of this goes so far as claiming divine status for Jesus, though, and apparently neither did the communities that produced those gospels.
John's gospel and Paul's letters do claim a divine origin and pre-existence for Jesus, attesting that this strain of thinking about who Jesus is co-existed and competed with the "messianic human" view in the earliest strata of Christian records. And we know plenty of other views were bubbling in the near vicinity -- the ones with whom Paul contended early on, the Gnostic view recorded in the first-century Gospel of Thomas, and more and more proliferating around the beginning of the second century and persisting until Constantine sponsored the Council of Nicea and put the stamp of approval on the full divinity of Jesus and his equality with God.
So if you think that you have to believe Jesus to be divine to be called Christian, then the majority of the earliest followers of Jesus who passed down the traditions we have today don't qualify.
Similarly with the afterlife. I'm not sure why Americans focus so strongly on this point -- again, students asked to define a minimum Christian doctrine set always mention life after death. Perhaps Nick hits it when he defines hell as what Jesus is saving us from. That's the Pauline view, certainly, but it's not in the synoptics. Moreover, it reflects a Hellenistic body-soul dualism, combined with a Zoroastrian God-Devil dualism, that is completely absent from the Hebrew scriptures -- the idea that one's soul has an eternal destination that renders earthly life merely prelude. Even Paul, who is to some extent fixated on Jesus' salvation from sin, death, and the devil (in the traditional formulation), regards the afterlife as something that will happen after the resurrection, at the end of time -- not your next stop after death. In any case, it seems that Jesus does have something to save us from other than death -- sin! I'd argue that this aspect of salvation is far more significant than immortality, but our culture seems fixated on the eternal reward stuff rather than the liberation from present bondage.
And while I have less of a gripe with a requirement to accept a heteronomous moral system, I wonder why it's identified as peculiarly Christian. After all, Jesus didn't give us a bunch of laws or moral precepts. His teachings, dare I say, were relative -- couched in relation to the interpretation of the law practiced by his opponents. Again, other than John's references to 'alethia, "Truth" (a Greek vision of an objective, transcendent ultimate reality), I don't see that a strict and knowable system of right and wrong, one that can be couched in commands and prohibitions, to be the dominant message of the gospels and the Pauline correspondence.
So what's the minimum? Very little, I'd say. Some central role for Jesus -- that's pretty much it. Anything more than that, and again, most of the early Christians are going to be excluded. That's how diverse the movement was from the first moment we gain sight of it in about 50 C.E., and in our search for a standard by which to measure our list of requirements, farther back than that we cannot go. We are stuck with diversity and we are stuck with a fuzzy, broad definition -- unless you'd like to put the starting point at Nicea instead, ignoring altogether the Jewish Christians who gave birth to the variety of Jesus movements, ceding the field to the Greeks and their philosophical formulations of Christ's divinity.