From his lecture “Contextual and Missional” at Urban Plant Life Conference in London, Tim Keller talks about the nature of a missional church:

A missional church gears absolutely every single part of its life–its worship, community, public discourse and preaching education–for the presence of non-believers from the culture surrounding it. A missional church’s congregation reflects the demographic make-up of the surrounding community–and therefore it gives non-Christian neighbors attractive and challenging glimpses of what they would look like as Christians.  A missional church’s worship is ‘evangelistic’ in the sense that it makes sense to non-believers in that culture, even while it challenges and shapes people with the gospel. A missional church’s people are outwardly focused, so involved in the local community, and so alert for every opportunity to point people toward Christ, that evangelism happens naturally through relationships.  Because of the attractiveness of its community, the contextual nature of its message, and humility of its people, a missional church will discover significant numbers of people always in the midst, ‘incubating’ and exploring Christianity.  It must welcome them in hundreds of ways.  It will do little to make them ‘comfortable’ but will do everything to make its gospel message understandable.