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 rediscipling of those who have formerly been evangelized, are tasks that go on through multiple lives and generations.
From Mission of Gods People The (Biblical Theology for Life) (p.285)
I wrote a blog post entitled Commission Impossible a while ago, which covers similar thoughts. It is probably worth mentioning that discussions about Bible Translation seem to be particularly susceptible to talk about “unfinished tasks” and the like – read my thoughts here.