Here is South African missiologist David Bosch on the nature of the church and mission from his milestone work Transforming Mission: Paradigm Shifts in Theology of Mission.
The Christian faith, I submit, is intrinsically missionary….This dimension of the Christian faith is not an optional extra: Christianity is missionary by its very nature, or it denies its very raison d’être.
Christian mission gives expression to the dynamic relationship between God and the world, particularly as this was portrayed, first, in the story of the covenant people of Israel and then, supremely, in the birth, life, death, resurrection, and exaltation of Jesus of Nazareth.
The entire Christian existence is to be characterized as missionary existence….The church begins to be missionary not through its universal proclamation of the gospel, but through the universality of the gospel it proclaims.
Theologically speaking, “foreign missions” is not a separate entity. The missionary nature of the church does not just depend on the situation in which it finds itself at a given moment but is grounded in the gospel itself. The justification and foundation for foreign missions, as for home missions, ‘lies in the universality of salvation and the indivisibility of the reign of Christ.’ The difference between home and foreign missions is not one of principle but of scope.
We have to distinguish between mission (singular) and missions (plural). The first refers primarily to the missio Dei (God’s mission), that is, God’s self-revelation as the One who loves the world, God’s involvement in and with the world, the nature and activity of God, which embraces both the church and the world, and in which the church is privileged to participate. Missio Dei enunciates the good news that God is a God-for-people. Missions (the missiones ecclesiae: the missionary ventures of the church), refer to particular forms, related to specific times, places or needs, of participation in the missio Dei.
The church-in-mission…is not identical with God’s reign yet not unrelated to it either; it is ‘a foretaste of its coming, the sacrament of its anticipation in history.’ Living in the creative tension of, at all the same time, being called out of the world and sent into the world, it is challenged to be God’s experimental garden on earth, a fragment of the reign of God, having ‘the first fruits of the Spirit’ (Rom 8:23) as a pledge of what is to come (2 Cor 1:22).
[Excerpts from David J. Bosch, Transforming Mission: Paradigm Shifts in Theology of Mission (Maryknoll, NY: Orbis Books, 1991), pages 8-11.]