First, let's review the content of "What is missions?"
So, from that article on "what is missions?", we see that a missionary is one who carries out the work of missions, that is, he or she is "a sent one", a person sent out from a local church (see Acts 13:1-4) in order to evangelize, disciple, and plant churches in another culture, or someone involved in training leaders in another culture, or someone whose role is facilitating getting the gospel into another culture. Romans 10:13-15 - ho shall they hear without a preacher? and how can they preach unless they are sent?" This principle is still ongoing, even though the office and special gifting of apostles is not going on today. (see below)
So, a missionary is somebody who is preaching and teaching and evangelizing into a new culture/people group that has not heard yet. "How shall they hear?" That is usually also called "pioneer missions" or "frontier missions", going where Christ is not known and worshiped as Lord. (see Romans 15:20-21) There are also missionaries in other cultures where the general culture is "reached", but they still need lots of teaching and discipleship and training of national pastors and help in areas of education and hospitals and technology and relief and development.
We saw that the English words "missions", and "missionary", come from the Latin word root of missio, which is a translation of the Greek words for apostle and apostleship. Since there are no more apostles today (see the above article), in the sense of the 12 apostles of Jesus and Paul and his missionary team. (with Barnabas, Luke, Timothy, Silas, John-Mark, etc.), the term "missionary" is a modern application of the continuation of applying the vision of sending out to preach the gospel and the command to extend the gospel into unreached peoples.
Usually we say someone is an evangelist who is sent out from a local church in order to reach people in their own culture, or a "church-planter" - someone sent out to a new area to start a church, but in the same culture.
In short: A missionary is someone who has been specially trained, called, and sent out, giving their life's work to cross cultures and advance the cause of the Gospel of Jesus Christ.
See also the article on "Thoughts on the Missionary "call", for relevant issues related to the question of "what is a missionary?".
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