Nowhere in the Bible will you find the term "closure." Nor will you find the phrase "finish the Great Commission." Nor will you find the number "2%."
You will not find "unreached" or "unevangelized" or "unengaged."
Moreover, you will not find "missionary" or "contextualization" or "chronological Bible storying" or "orality" or "Bible smuggling" or "creative access" or gasp "tentmaking."
I'm being completely honest. Something I think is incredibly important - something I advocate for, something I passionately beg the church for - is not exactly in Scripture.
These are technical terms that missionaries have created, defined, written up, passionately commended, argued from bits of Scripture, debated, called each other names over.
"Charity" you will find. "Gentiles" you will find. Even "ethne" you will find. But not closure.
Not finish the task. Not reach the unreached. Not 10/40 Window.
So why hold to them?
I do, partly because once upon a time I was given a vision of the unreached, one that marked my life, one that I do not fully understand but cannot argue with.
And, more to the Biblical point, because I believe that in Matthew 28 (and other places) Jesus gave us a task, and having given it to us, he expects that we should finish it. I tell my children to clean the kitchen, and I expect them to do the job. If I, as the head of house, expect this of my children, should I not expect the same of the one who is head of me?
Now, my idea of "finishing the task" is perhaps different from others. That's okay. I'll argue for mine, but with genteel charity and kindness (I hope). In my idea, I think "unreached" is important (and unevangelized, and unengaged) because of two logical things: first, that there are people who have not yet heard the Gospel (thus my task is not finished); second, because of the Paul's strategy of not preaching the Gospel where it is known.
And I think the other terms are important as tools, tactics and measuring rods.
So when someone argues with you--or me--that closure is not Biblical, don't argue. Agree. It's not. It's just a useful tool for defining part of the space, task, responsibility that Christ has called the church to, and the gap in the wall toward which I (and others like me) go. You may have another gap to fill, and that's good. The Body needs all parts.