I don't know what is required to be a "true" Christian, but I don't have any problem with John Spong being "Christian". I haven't read any of his books, although they've been suggested to me a few times.
Regarding the interview, I think it's interesting that he and I seem to share a kind of conviction that there is a challenge facing Christianity in that traditional theology, or that many elements of the traditional myth, are no longer believable. "Pious tripe" he calls it, which might be a bit harsher than how I would put it, but the conviction is similar. So he writes:
"I don’t think you can locate it, and that’s the problem with writing this book. I only have a language bounded by time and space. I’m trying to discuss something that’s not bounded by time and space. I don’t have a language — all I can do is point to it. So I couldn’t go through religious symbols, because they are all located in a supernatural being external to the world who’s sitting in some sort of judgment. And that’s not the God I can communicate anymore. But I don’t think that means there is no God. We need to reconceptualize the human experience of God. The more deeply we understand life, the more deeply we understand God as the source of that life . . . and that’s the direction the Christian faith is going to head in the 21st century."
My reaction is that I agree in some ways with his view of the problem. There is a theological problem with the way many Christians hear the word "God".There are difficulties making sense of the idea of a super-Being who, being separate, creates the universe with perfect metaphysical freedom and omnipotence, with salvation as a purely future judgement leading to heaven or hell, and etc. But his perception of the problem is that it means he can no longer use religious symbols, whereas my discovery has been that the very symbols themselves go much deeper than those theological problems, which are in some ways quite modern. You can find more subtle theological reflection on the "human experience of God" in Christianity. I'm often quoting examples in posts here.
So I don't entirely agree that it's necessary to abandon traditional religious symbols. But I can sympathize with the desire to do so. There are many conversations about "God" that I feel would be easier without the baggage of a lot of presupposed theological conception. But symbols are more than just words denoting conceptual frameworks, they are richer than that. Tradition is not merely the recitation of a rote formula, but the living vision of each generation which has received whatever wisdom may be handed down (tradition: from tradere, to hand down). It's more like a garden than a text, and the wisdom of traditional Christian symbolism is not, for me, just in a doctrine or a creed, but in the testimony of that experience of God which Christians have preserved.The soil has been prepared but we have to grow new plants, so to speak. Even so, I think it's good and useful to have roots and be aware of them, as much as it is also necessary for Christians in the 21st century to give new expression to the experience of God, and to reconcile that experience with the modern world.