Have I posted this before? If so, I don’t apologise, it’s worth reading twice!
“We should not treat the Great Commission as a ticking clock, just waiting for the last people group to ‘hear’ the gospel before the Lord is, as it were, permitted to return. That kind of thinking has transformed it into a ‘job to complete’, ‘an unfinished task’. But with its command to disciples to make disciples it is a self-replicating mandate that we will never ‘complete’ – not in the sense that we can never reach all the nations (we can and we should), but in the sense that the making of disciples, and the re-discipling of those who have formerly been evangelised, are tasks that go on through multiple lives and generations.”
 Christopher Wright, The Mission of God’s People : a Biblical Theology of the Church’s Mission (Grand Rapids Mich.: Zondervan, 2010), p. 285.