Hey Kristin--
I liked your post, because it asks one of the big questions that people who study Christian formation seem to be talking about right now. The language that they often use is from mathematics: is the church a "bounded set" or a "centered set"? The bounded set idea, which is a more traditional view, names certain beliefs and behaviors that form a boundary between who is, and who is not, a Christian. The centered set approach, as I understand it, focuses on our movement toward the center of faith (Jesus), which takes place over one's lifetime, and is more willing to be a little fuzzy on the question of what constitutes being "in" and "out." I like that metaphor of moving towards and ever deepening center, and I wonder if that speaks to you.