Q. I wonder why missionaries should be sent to Europe.
First, let's ask - should we send no workers to Europe, at all?
A lot of people who advocate work among the unreached say we should preference sending workers to the 10/40 Window over sending workers to Europe. What gets lost in the soundbite is the reason: there is an enormous imbalance between missionaries sent to the 10/40 Window and those sent to "Christianized" places (like Europe and Latin America).
Divide the world up into Worlds A (unevangelized), B (evangelized non-Christian) and C (professedly Christian), and look at the missionary resources spent on each: roughly 90% is given to World C, 9% for World B, and a smidge for World A. This is an enormous imbalance.
Why does this imbalance exist? A few reasons:
These aren't bad reasons. They are simply reasons why it's easier to send workers one place, and harder to send them to another. It's very difficult to send workers to Afghanistan at all, for example--so the church can hardly be blamed for not sending many workers there.
Should we send no workers to Europe? No. But we should, insofar as it is possible, work very hard to correct the imbalance.
Can we not send as many workers/resources to Africa and Asia as we do to Latin America and Europe? Could we not send as many per capita (e.g. if we send 1 missionary per million to Europe, can we send 1 per million to Africa and 1 per million to Asia - knowing this means more workers for Africa & Asia)?
Now, let's consider the inverse question: why should we send workers to Europe?
I define a "missionary" as one who is sent on a mission; specifically, to plant the church where it is not.
Some note, rightly, we are not commissioned to "plant churches" but rather to witness, evangelize and make disciples. The church is built when disciples are made. While true, the soundbite can conflate the individual congregation with the larger Church. When we make disciples, those disciples gather together, and their gathering is a church, which is a local expression of the Church.
For witness to be offered, the Gospel to be shared, and disciples to be made, the Church (in some form, whether local believers or foreign workers) must be present. The missionary task is to plant the Church (which is a far different matter from starting individual local churches). While missionaries may witness, evangelize and make disciples, local believers can do this far better. So the missionary really should only do this to the extent required to get to the point where local believers are doing it. At that point, the missionary task for that place/people is finished, and the local responsibility begins.
Europe is controversial. The "Church" is arguably present. Depending on whether you count Anglicans, Catholic, Orthodox, and Protestants or not, it's either present in large numbers or small. Whether the evangelization of nominal, cultural Christians is a missionary task or not is a matter of no small argument. For me, I say "no." But, if you argue the growth functions of the church (witnessing, evangelism, disciple-making) are not being done, then this would (to my mind) justify the deployment of missionaries.
However, there is another reason to send missionaries to Europe: to work among the many thousands of non-Christians (e.g. atheists, agnostics, Muslims, Hindus, Buddhists), whether local or immigrant (and there are obviously many thousands of immigrants coming in to Europe). London, Birmingham, Paris, certain sections of Germany, parts of Eastern Europe, and Turkey (if you count Turkey as part of Europe) are obvious locations for this kind of work (as are other places I'm sure many could name).
Europe is also often a good proximity base to support work being done in other places (e.g. broadcasting into the Middle East, or translation support, or follow-up for Internet or broadcasting-based programs, etc).
The most important reason one could have for sending workers to Europe is that God called that particular person/agency to go. If God says 'go,' don't listen to me say 'stay.' (Just make sure that God really called you.)