The word ferment can be either a verb or a noun, so I find it to be an interesting word choice for the work to which the Church is called. It can refer to the process of fermentation, as in making bread rise or turning wort into beer. Jesus spoke about yeast that "leavens the whole loaf" and also about needing new wineskins for new wine. As a verb, ferment can also mean "to incite or stir up trouble." It's interesting to think about the work of the Church in this way and we can look to ways the Church responded to the threat of Nazi Germany (in places like Le Chambon, or the Confessing Church) or in base communities in Latin America (that have so clearly left a mark on the current pope) or to the resistance in South Africa under apartheid to see how and why this "stirring up trouble" meaning of ferment matters so much.
As a noun, it can refer to both of these same things: agitation and excitement among a group of people, or as the enzyme or fermenting agent that leads to that process of fermentation.
What does it mean to imagine the Church as a "fermenting agent" within the larger cultural context in which we find ourselves? The whole world doesn't need to become light, or salt, or yeast. As the late Krister Stendahl liked to put it, the Great Commission is not about making the whole earth into a salt mine! But knowing what a little light in the darkness can do or what a little salt can add to a steak or what a little yeast can do to some flour and oil and water to make the whole loaf rise helps to clarify the work to which the Church is called.
Alan Kreider's book is helpful in clarifying this mission. He's focused on the "why" of what we are called to be about. Quoting Michael Pollan, Kreider notes that "A ferment generates its own energy from within. It not only seems alive; it is alive. And most of this living takes place at a scale inaccessible to us without a microscope." Kreider goes on to explore the ways that the early Church focused on habits, on forming Christians, on inculturation which he notes has two aspects: the indigenizing principle by which God is found at work in the world and the culture is celebrated and the pilgrim principle by which the dominant culture is critiqued and challenged based on the values and norms of the Gospel.
Kreider is not the first to suggest that this both/and relationship to the culture makes Christians into resident aliens, paroikoi in Greek. Out of love for the culture in which we find ourselves, we also point toward the necessary healing of that culture as a patiently fermenting presence within that culture.
In my experience, this kind of nuance is often lacking in many of our congregations. Episcopalians are likely to look and act just like our neighbors; the Amish are likely to look very different. But this dynamic interplay, as Kreider calls it, gets lost. So how can we become more intentional about this? What are the indigenizing aspects that need to be claimed in our own world - those aspects of our culture that should be celebrated and claimed and seen as ways that God is already at work in the world? And what aspects of the culture need to be critiqued and challenged by a pilgrim people? I suspect that we won't all immediately agree on how we respond to such questions but they might allow the conversation to unfold in more intentionally ecumenical ways.