Author Ken Callahan offers a very good answer:
In the mission growth movement, the compelling question is, "Who is our mission?" That is, "Who is God inviting us to serve in mission?" God invites us to a theology of service, not a theology of survival. God invites us to a theology of mission, not a theology of maintenance. God plants around us people with whom we can constructively be in mission.
From "A New Beginning for Pastors and Congregations"