Missiologists have increasingly been drawn to this phrase to express the conviction that mission is not the invention, responsibility, or program of human beings, but flows from the character and purposes of God. Historically, the term mission was first used by theologians to refer to the acts of God, rather than the activities of the churches.
God is the Missionary, who sent his Son and sends his Spirit into the world, and whose missionary purposes are cosmic in scope, concerned with the restoration of all things, the establishment of shalom, the renewal of creation, and the coming of the kingdom of God, as well as the redemption of fallen humanity and the building of the church. Mission has a trinitarian basis and is theocentric rather than anthropocentric. Mission is defined, directed, energized, and accomplished by God.
For church planting this has considerable significance. First, the inevitable interest in internal church structures which characterizes church planting initiatives, as plans are developed for the formation of a new congregation, must not subvert the primary focus on the mission to which this new church is being called. Missio Dei is toward the world rather than the church.
Second, the broad scope of missio Dei must not be reduced to evangelism or church planting. Church planting is legitimate only if set within a broader mission context. Church planting can too easily embody a limited vision of mission that concentrates on one or two aspects of this mission (usually evangelism and church growth) to the neglect of other vital aspects (including working for justice and peace within society, concern for the environment, and engagement with culture).
(From Stuart Murray via)