This is a great post from Missions Launch. ML is a great blog that is full of information for those involved with reaching the nations. They "support those people by providing information, resources and support as they prepare to embark on an adventure of a lifetime."
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That is a great question. First let me tell you what it is NOT. It is NOT groups of people in your neighborhood who aren’t Christians or who are or who don’t believe. Unreached is much more than that. Unreached not only are not Christians, but they have almost no way at all to EVER hear the gospel because of their language and location and history. Also, unreached people groups are specific ethnic groups with their own languages and culture. For example, Iraq is might be considered an unreached country, but it is not an unreached people group. Instead you would say that the Kurdish people within Iraq are the unreached people group.
Unreached groups don’t have any significant numbers of Christians that can tell the others in the group about Jesus. Unengaged groups don’t even have any outside missionaries working there or planting any churches. Basically, if you are in an unreached people group, you are unreached with even the words of the gospel and almost any Christians. Your odds of ever hearing the gospel or meeting even one Christians in your entire life are slim to none.
I once was talking to a friend about an outreach program to an unreached people group in a closed country (where there are almost zero Christians.) He asked if we were partnering with any local churches. Clearly, his question shows that he hadn’t understood what “unreached” meant. In an unreached area, there are no local churches. Therefore, someone has to be the first to go there to tell them even who Jesus is. In some locations, it might be even necessary to explain who God Himself is.
This is a great video from Pioneers that explains a bit more…







