Missionary researchers and strategists divide the world into roughly three spheres (although these may be measured differently): first, those who are 'Christian' (or who would profess in some way to be part of Christianity); second, those who are not Christian, yet are 'within reach' of the Gospel; third, those who are not Christian and out of reach of the Gospel. "Unreached" and "unevangelized" and "unengaged" are different ways of measuring which of the latter two spheres people are in. When we speak of a "Christian" sphere, we may be referring to an individual, a place, or a people group. Most of the time, when we call a large mass of people (either a people group or a place) "Christian," we simply mean the majority of the people within that mass are Christians.
Within a large mass of majority-Christian people, there can be several different types:
The presence of non-Christians and anti-Christians (and even large numbers of 'bad' Christians) doesn't make a particular place a non-Christian sphere. When researchers refer to a place as "Christian" or "Christianized," we simply mean that the work of planting the church has been well founded. Pretty much everyone in a place knows a Christian, whether they like them or not, whether they are good or bad. In the span I have described above, "Good" Christians and "Anti" Christians tend to be in a tug-of-war for the middle two spheres. The skills required in this "tug of war" are not precisely missionary skills, however.
On the "Good+Bad Christian" side of the scale, the work that needs doing is the work of a Pastor. Last Sunday, the pastor of our local church shared how he came to faith when he was a teenager. "I had been baptized, I was in church, I was a 'good' Christian, but I didn't have a personal relationship with Jesus Christ." I won't argue that he was actually a Christian who needed revival: he clearly recognized he was not. But how did he get to teenaged without knowing he needed a personal relationship with Christ? What he needed was not a missionary - he needed, in my estimation, better pastoring, better disciple-making. (I am not trying to be judgmental here, although it is a judging sort of statement, made possible by the evidence offered.)
On the "Non+Anti Christian" side of the scale, the work that needs doing is the work of an Evangelist (and sometimes Apologetics). Pastors are oriented toward those already in the church, Evangelists are oriented "outside" the church to those who would never come into a church building at all.
These two work together, however: I am reminded that when Billy Graham came to do a crusade in a city, the first groundwork was to get the churches together in a followup network. When someone is brought into the Kingdom by an Evangelist they need to be discipled, and sometimes this is done by the Evangelist and sometimes not.
Neither of these two sides of the scale, however, need a Missionary.
In Romans 15:20, Paul makes his famous statement: "It has always been my ambition to preach the Gospel where Christ was not known, so that I would not be building on someone else's foundation."
When a large share of a place is Christian, good or bad, a foundation has been laid.
If you want to go some place where there is a large nominally Christian community and evangelize or pastor there, that's not a bad thing. There were times in the New Testament when people were sent to help new believers (Acts 8:14-17, in the wake of Philip's evangelistic ministry, is an example; also Acts 11:19-26). You aren't necessarily disobedient to the call of Christ. But it is not the missionary task. The missionary task is very different. (I realize we might disagree on the definitions, but to me a core part of mission is Romans 15.)
When there are a lot of 'bad' or 'nominal' or 'apathetic' Christians, better pastoring is needed. Where there are a lot of non-Christians around a church, better evangelism is needed. Where there are no Christians whatsoever (or a minuscule portion of the population), missionaries are needed.
So let's be clear what we are recruiting for!