This is a very good question! Ultimately only God knows how He is measuring when a people group or nation is “reached”; and when “the full-ness of the nations” has come in. (Romans 11:25)
Every culture still has lost people in them, even those that are considered “reached”, that have a substantial among of believers and churches.
One way to see the world is the reached vs. the unreached. Another way is too divide up the world into the categories outlined in one of the previous articles. (see here)
Is a previously unreached people group reached after they have one convert?
What if a country or people has a few disciples, but they are scattered, persecuted, and don’t really have any churches yet?
What if an unreached people group has a few house churches, but cannot really do much yet by way of influencing their culture and society for the gospel?
Some mission organizations said that a people group is reached when they have the resources in their own local churches and leadership to reach out to their own people without foreign missionaries doing the work. That’s a good practical definiton.
Another problem is that some missions statistics see the western world and countries that were previously “Christian” in culture, and so, they are considered “already reached”, but that is not accurate, either.
Every generation is responsible for the Great Commission.
We cannot say that Turkey has already been reached, just because in the past that land had the gospel. The people of the days in the New Testament and first 800 years that lived there were not Turks. They were Byzantine Greeks, Galatians, Phyrgians, Armenians, Romans, Cappodiacians. The Turks started coming there in the 900s AD and defeated the Byzantine Empire at the battle of Manzikurt, near Van in 1071 AD (in the east near the border with Iran today). The Arabs had converted the Turks to Islam in the 900s and they became the military force for the Muslim world soon after that. The Turks originally were from Central Asia, areas today known as Turkmenistan, Azerbaijan, Uzbekistan, Kazakhistan, and Kirgistan.
Some zealous evangelicals emphasized just evangelism into pioneer areas too much, (the gospel of the kingdom must be preached as a testimony in all nations” – Matthew 24:14) without sound teaching, and taught things that seemed as if they were saying, if one person gets saved in an unreached people group, then that group is “reached”. That doesn’t seem right. Also, some people emphasize evangelism over discipleship and teaching and that is not right either. The nations must be discipled and taught well. (Matthew 28:19) The Greek of Matthew 28:19 does not mean, “disciple a few people in each nation”, rather it means “disciple all the nations” – each nation is to be discipled with sound evangelism and teaching.
Some groups and churches invoke Matthew 24:14 as if we can, by our missions efforts, actually bring about the second coming of Jesus. I don’t think that is right either, since God is sovereign and only He knows when all the elect will be saved; but it is proper to show that the reason why Christ has not returned yet is because there are still many people out there in all nations who have yet to repent, and who will repent when the gospel is preached to them. (2 Peter 3:8-15)