The question of phases of life, of passages, and how we transact the journey from here to there has become a common one in our culture. The idea really comes from one of the great early works in the beginnings of anthropology. When Arnold van Gennep's “Rites of Passage” addresses this question, he describes the place between the state before and after as “liminal,” from the Latin word for “doorway” for in-between-ness. The word has come to be used for people or times, which are transitory, which aren't so easily categorized. A later anthropologist named Victor Turner came to apply the idea to liturgical practices, which didn't fit into the structure or hierarchy of ordinary life, and so were located in the gap. Festivity, play, ecstasy, and imagination are all associated with the liminal. Think Mardi Gras, New Year's Eve, or a graduation party. The idea has been applied powerfully to things religious. Figures who are liminal may be throwbacks to an earlier era, or non-conformist, coming as they do from the margins. But they also have the power to introduce new ideas the society can only assimilate gradually or later. Think John the Baptist, Gandhi, or St. Francis.
The classic applications in Christian life are things like a baptismal or ordination retreat or Cursillo. The time is set apart, as is the place so that you can by grace come to be a new person. In ordinary Church life Sunday is a different day planted in the midst of our earthly days- this is the argument for keeping a more distinct Sabbath rule. Even things like vestments spired (mountain like) buildings have a liminal aspect. Of course one paradox of the faith is that God the signs of whose presence are liminal actually owns it all!
I have been thinking about how the word does inform the role of bishop. A diocese is not best understood as a hierarchy or a voluntary non - profit, but rather as a gathering at best, or at times just a collection, of Christian congregations. It is a clump of worshipping cells, and this peripatetic liminal character the bishop appears on a certain Sunday as a sign of the Church global, historical, eternal. The event is supposed to be odd, though you hope he won't be too much so.
Similarly we might say that renewal, missionary, or theological movements are liminal - they intrude with a possible future from God into ordinary Church life. In the same way a diocese with a particular theological perspective has a witness which is liminal - it recalls parts of the past too easily forgotten and points to a renewed kind of life. Those who have lived at the edges or interstices have contributed unique things, which, the ordinary Church could not yet receive - the Monasteries, the Mennonites, even the Quakers. This helps us understand positively a vision, which for now seem out of step with the times.