When Karl Barth addressed the Brandenburg Missionary Conference in 1932 he introduced a missiological perspective which has determined the shape of mission theology in every part of the Church.
"Must not even the most faithful missionary, the most convinced friend of missions, have reason to reflect that the term missio was in the ancient Church an expression of the doctrine of the Trinity-namely the expression of the divine sending forth of self, the sending of the Son and Holy Spirit to the world? Can we indeed claim that we do it any other way?"
Barth cuts through soteriological or eschatological consideration to bring us right back to the Source of mission. It is not that 'Salvation is like this therefore mission should be like that.' It is not that 'The End will be like this, so mission should be like that.' No, the real argument is that 'God's being is like this, therefore mission should be like that!' There are missions because of the missio Dei - because God is a sending God. In Himself, in eternity, God's being is a being of outgoing love. This is the Fountainhead for mission.
David Bosch has memorably put it like this:
To participate in mission is to participate in the movement of God's love toward people, since God is a fountain of sending love.
This insight has been picked up by all wings of the Church, from the conciliar to the Anabaptist, from the Roman Catholic to the evangelical.
More important than all this consensus however is the bible's own testimony.
Consider the Johannine 'great commission':
As the Father has sent me I am sending you. (John 20:21; cf 17:18).
We ought to take that little word 'as' with full seriousness. In the same way that the Father sends the Son, so the Son sends His church. Let us ask, how has the Father sent the Son?
Lest we be Arians we must acknowledge that the Son's generation from the Father is not a mere product of the Father's will in time. It is rather an eternal begetting that is of the very essence of the eternal Godhead. There is not a God and then a sending. There has only ever been a sending God - the missio Dei. Both Father and Son are eternally constituted in these relations of Sending and Sent.
The Son's being and act is a being and act found and expressed in the Father's sending. The Son's own life is a life in mission. This has always been true in eternity and it was made manifest in incarnation.
Christ's most common self-identification in John is as the One sent from the Father. And His most common articulation of His mission was always to do the will of His Father - a will expressed in thoroughly evangelistic terms - e.g. John 3:16; 4:23; 6:29; 6:38-40. Christ is sent as the world's Saviour, the One who seeks worshippers for the Father, who glorifies the Father in His saving death and only then says 'it is finished' (John 19:30).
Therefore, because Christ's being is a missionary being, so His activity is a missionary activity.
On the cross, the true being and glory of the Son was manifested, and in Him the glory of the triune God (e.g. John 13:32; 17:5). Here was demonstrated Christ's obedience to the Father and, at one and the same time, His love for the world. Christ's being and act are laid bare at Golgotha, and shown to be a missionary being and act.
Therefore, returning to John 20:21, we see the continuity of Christ's mission with ours. Just as Christ has His being in sent-ness for the world's salvation, so does the church. We have received a commission that was passed from the Father to the Son in the depths of eternity. Our missionary activity finds its origin not in any human enthusiasm for witness but in the being of God. And our sent-ness for the salvation of the world is not only our activity. It is, like God's own missio, constitutive of our very life.
'The Christian community is not sent into the world haphazardly or at random, but with a very definite task. It does not exist before its task and later acquire it. Nor does it exist apart from it, so that there can be no question whether or not it might have or execute it. It exists for the world. Its task constitutes and fashions it from the very outset. If it had not been given it, it would not have come into being. If it were to lose it, it would not continue. It is not then a kind of imparted dignity. It exists only as it has it, or rather only as the task has it. Nor is it a kind of burden laid upon it. It is the inalienable foundation which bears it. Every moment of its history it is measured by it. It stands or falls with it in all its expressions, in all its action or abstention. It either understands itself in the light of its task or not at all.' (Karl Barth, Church Dogmatics, IV/3, p796.)
'[The task of the Church] is no less, no more and no other than the ministry of witness required of it and constituting it.' (Karl Barth, Church Dogmatics, IV/3, p834))