A footnote from "Exclusion and Embrace: A Theological Exploration of Identity, Otherness, and Reconciliation", Miroslav Volf.
The metaphor of the door is helpful insofar as it implies a necessary demarcation, but it is also misleading insofar it suggests a sharp and static boundary. In analyzing the category "Christian", missiologist Paul Hiebert suggests that we make use of the mathematical categories of "bounded", "fuzzy" and "centered sets". Bounded sets function on the principle "either/or": an apple is either an apple or it is not; it cannot be partly apple and partly a pear. Fuzzy sets, on the other hand, have no sharp boundaries; things are fluid with no stable point of reference and with various degrees of inclusion - as when a mountain merges into the plains. A centered set is defined by a center and the relationship of things to that center, by a movement toward it or away from it. The category of "Christian", Hiebert suggests, should be understood as a centered set. A demarcation line exists, but the focus is not on "maintaining the boundary" but "on reaffirming the center"
Hiebert, Paul G. "The Category 'Christian' in the Mission Task." International Review of Mission 72 (July 1983): 421-27.