A proper, biblical ecclesiology looks at everything the church is and does in relation to the mission of God in the world. The church does not exist for itself, but for participation in God’s mission of reconciliation. “Mission” is not just an activity carried out by special people in faraway places. Mission is the character of the church in whatever context it exists.
This hasn’t always been the way Christians have thought about the character of the church. In Christendom (where church & nation/culture were hand-in-glove, and it was assumed that almost everybody was Christian somehow), the church’s mission only related to cultures other than the dominant culture.
This was especially the case in Europe and North America. But Christendom is dying. Our context in North America is more like the New Testament context of the church, where the church is on the margins, not at the center of society. The mission field is right around us, as well as around the world. We can no longer assume (if indeed, we ever should have assumed) that everyone around us is Christian.
Nor is a missional church simply a congregation with a mission statement. All kinds of organizations have mission statements, and not all of those mission statements are aligned with God’s purposes in the world.
A missional church is a church that is shaped by participating in God’s mission, which is to set things right in a broken, sinful world, to redeem it, and to restore it to what God has always intended for the world. Missional churches see themselves not so much sending, as being sent. A missional congregation lets God’s mission permeate everything that the congregation does—from worship to witness to training members for discipleship. It bridges the gap between outreach and comgregational life, since, in its life together, the church is to embody God’s mission.
—Lois Y. Barrett in Treasure in Clay Jars: Patterns in Missional Faithfulness