From time to time I am asked why we send missionaries to Europe when it has had a chance to hear the gospel and there are still populations that have not. It is a good question but like so many questions it is not an either/or but a both/and. Let's think through some of the issues involved.
First, in the great commission, Jesus made it clear that we are to go and make disciples of all nations. In Acts 1:8, he said, you will be my disciples in Jerusalem, Judea, Samaria and to the uttermost parts of the earth. The implication is that we are to go wherever the gospel is scarce. Today, that also includes populations that have had the gospel in the past but do not today.
Second, what does it mean that we should go only where the gospel has never been preached? Some of the earliest places to receive the gospel were North Africa, and Turkey. Does that mean we don't go back today when Christianity is almost non-existent? Today, these would be considered part of the critical 10-40 window. Yet one cannot say they have not had the gospel in the past. Both were leading centers of Christianity in the early church. For that matter, Constantine's whole empire had the gospel at one time. Yet there is a tremendous amount of difficult mission work going on in these places today because today they are unreached populations.
Let's apply that same logic to Europe. We in the west are the recipients of the Reformation in Europe, as are all the nations to whom the west has sent missionaries. But remember that much of Europe was not evangelized by the Reformation for within a hundred or so years of the Reformation, European regions were either almost fully Catholic or fully Protestant. Much of Europe was not a recipient of the Reformation message or if they were, not for long. This was certainly true of places like Poland and Spain (where the current evangelical population is below 1%). The Reformation brought with it a counter Reformation of the Catholic church along with conflict, wars and finally, divisions where only Protestantism or Catholicism became legal.
Of course, even in places where the Reformation did have enormous impact (England, The Low Countries, Scandinavia and Germany) there is very little left of its influence apart from beautiful churches which are mostly empty on Sundays. Just as the Mongol hoards brought Islam to North Africa and Central Asia, so secularism has brought spiritual deadness to much of Europe. Both were equally deadly to the gospel.
Finally, we need to think about how the world has changed and who actually resides in the great cities of Europe. Go to any major city on the Continent and you find people from everywhere in the world - including great numbers of those we would call unreached today from places like Iran, Iraq, North Africa, and nearly every country on the globe.
If one goes to Germany, for instance, they will find a resurgence of evangelicals among the Iranians there. The point is that if you want to reach unreached people, the cities of Europe are prime opportunities, not only for Europeans but for immigrants who are coming legally or illegally from all over the world.
One of the significant churches in Stockholm (as secular a city as one will find) is New Life Church where on any given Sunday you will find about 800 Christ followers, half Swedish and half from the rest of the world (services look like the United Nations). They want to plant some 20 similar churches in Stockholm.
All of the immigrants of Europe have ties back home including family so introducing them to the gospel has a huge ripple impact around the world. By definition, if you want to reach unreached populations, the cities of Europe are central to that mission.
So my answer to the original question is that we must send missionaries wherever the gospel is scarce. That includes those who have never heard, those who heard a long time ago and those countries that heard in the past 500 years but where the gospel message has again become scarce. That, by the way is why we plant churches aggressively in the Untied States as well even though the gospel has been known in this country since the first immigrants from Europe appeared in modern times.
It is not an either/or but a both/and.