I quite like this quote from Lesslie Newbigin
The western world has had to be recognised once again as a mission field, and the churches have been compelled in a new way to define their nature and mission as parts of a divine society distinct from the wider society of nations in which they live and all these factors have contributed to developments in the field of theology in the direction of a missionary understanding of the nature of the Church itself. The truth that the Church is itself something sent into the world, the continuation of Christ’s mission from the Father, something which is not so much an institution as an expedition sent to the ends of the earth in Christ’s name has been grasped with a new vividness. Among those who have reflected about these matters it becomes less and less possible to speak of the missionary task otherwise than as the embassage of the whole people of God to the whole world.
It’s a very short quote, but it has potentially far reaching implications for churches and mission agencies.
You can find the full quote in his paper, The Mission of the Triune God.