—Persons interested in, or inspired by, what is to them the “legend” of Jesus, but who have no tendency to think there really was a person behind the name, do not seem to me to be fishing in waters even close to the consensus meaning of “Christian.”—
Why? Almost certainly because you wouldn’t buy their theology. But that’s okay: they don’t buy yours. But don’t pretend that their agnosticism on a historical Jesus is born out of apathy. It’s born out of honesty, combined with a sense that the metaphorical can sometimes be far more important insight into the mind of god than the historical. Some people don’t believe that god speaks through history: that his messages are entirely spiritual, and the Bible is a collection of many people’s attempt to understand these ideas.
—Do you mean “humanistic” in the sense in which it is linked to agnosticism/atheism?—
No, I mean, as in human. As in, Christ was human, with faults, fears, and passions. That he had an actual character instead of simply being a dull collection of intrinsic maximums: a perception which largely became popular only in the modern age (but now, apparently, is so prevalent that people cannot seem to concieve of it being any other way).
—It is clearer to express this in reverse. A Christian can NOT believe that Jesus was just a wise teacher and one heck of a great guy.)—
Why not? A great teacher, and a heck of a guy who saw the mind of god, and whom at the end god chose to make known his grace and mercy through. Not all Christians buy the specific pagan sacrifice story that some do that requires that Christ be perfect and sinless (as if that somehow made any sense of how killing him accomplishes anything).
—Not familiar with the views of Spong.—
You don’t seem very familiar with much of Christianity beyond a rather small spectrum of modern ideas. I would really suggest you read some books by Shelby Spong, or at least Paul Tillich.
—If your objection is to “person-to-person,” sans the mediation of the Church and its priesthood—
That is exactly my objection. The idea that each individual can commune directly with god on a personal level is a Protestant idea. There are substantive theological differences in the sort of relationship one has with Christ, and even if you massage the meaning a bit, “person-to-person” simply doesn’t fit Catholic theology very well at all.