Joshua Project uses the terms "unreached" and "least-reached" to mean the same thing. The terms are used interchangeably. An unreached or least-reached people are a people group among which there is no indigenous community of believing Christians with adequate numbers and resources to evangelize this people group.
Ralph Winter words this definition a little different and I quote him for clarification: “Unreached peoples: a group among which there is no indigenous community of believing Christians able to evangelize this people group.” (Unreached Peoples: The Development of the Concept, International Journal of Frontier Missions, Vol. 1, No. 2, 1984).
While these and other definitions are not complete in themselves, they give us a framework in which to position our efforts so that such people might be reached. As this ministry (NFI) moves forward in 2015, our objective is to effectively impact church plant and church renewal so there is a fresh moving of the Holy Spirit through individuals in the church that carries the gospel to the unengaged and unreached people groups of the world. Paul’s ministry was toward that end so that “the word of the Lord sounded out from you” (1 Thessalonians 1:8). When individuals are spiritually mature (healthy), so will be the church. Then expansion of the gospel will be spontaneous!
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