It is probably best summed up by the quote from Rowan Williams that: It's not that the church of God has a mission, rather that the God of mission has a church.
Our role, as God's people is to partner with him in the mission he is orchestrating in our world. We were formed for this mission and called into this mission. Roxburgh and Boren put it this way:
Mission is not something the church does as an activity, it is what the church is through the mystery of its formation and memory of its calling.
Introducing the Missional Church p45
Taking on such a perspective requires a re-imagining of what church is and how it functions. It is not about developing new programmes of things to do, it is about a new imagination about being missional, of partnering with God wherever and whenever he is at work in the lives of those around us. It is important to understand that this is not about redefining the church in terms of evangelism. Missional does not equal more evangelistic in the traditional sense of those words. Evangelism is part of mission, but it is not the whole of mission.
Over the centuries I believe that the church has lost this missional edge. It has replaced being the church with doing church, it has replaced a relationally based community of followers with a programme based community of adherents.