Recent conversations have reminded once again that the terms we use to define the task are not Biblical terms. They are barely Biblical concepts. We try to drive our understanding from Scripture, but these measures are always lacking in some way. It seems very clear (at least to me) from Scripture (Matt 28 and 24 are our go-to passages, always, but Rev 7:9 and pieces from the OT etc fit in as well) that we have been given a task. This task involves both proclamation of the Good News and disciple-making. There's even a big, tantalizing, mysterious carrot for us: Matthew 24:14 can be read to indicate when the task is done, Jesus comes back. (Maybe.)
We know what Scripture records of how Jesus did things, but even Scripture tells us the record isn't complete. (End of John.) And we don't want to get "too locked in" to things we shouldn't be locked in on. Jesus didn't do Twitter - but then Twitter wasn't around in those days. Would he have done Twitter if it had been? Eh...
Here's an analogy: it's as if I headed out the door, and as I headed out the door, I told my kids to do the dishes before I got back. (Rough analogy.) I didn't tell them precisely HOW to do the dishes. So, what do they have to judge whether the task I have given them is complete? They have basically how I've done the dishes in the past (my example) and how I've shown them to do the dishes in the past.
When I said "do the dishes" did I mean just the ones in the sink? Did I mean to scour the house for every last dish? Did I mean to re-wash all the dishes in the cupboard? It's only obvious if you know me - and even then, there can be some ambiguity.
When we define "closure" and "unreached" and "unevangelized" and "the 10/40 Window" and "non-Christian" and all these terms... we are getting into messy and murky waters. It's not that we shouldn't try and define the task - definition precedes doing and measuring. But we should realize that in many ways we are making up our own measures to try and describe "the things we have seen and heard."
(Really, we're describing the things we see and hear as recorded in Scripture...)
People will quarrel and argue and fight over these definitions when maybe we should hold them very loosely.
My general rule of thumb is: let's take a definition and do it. And if Jesus doesn't come back, let's just take another, deeper, definition, and do that. And let's allow for mystery and muddiness and murkiness in the waters while we do, and be humble and charitable about our work.