The biblical mandate that the Church take the gospel to every people group on earth, making disciples who make disciples. From Latin missio, "a sending." The OT laid the foundation (Genesis 12:3 — "in you all the families of the earth shall be blessed"; Psalm 67, Psalm 96, Isaiah 49:6); the NT brought the explicit commission (Matthew 28, Mark 16, Luke 24, John 20, Acts 1); the book of Acts and the epistles record the first Church-age outworking. Evangelical missions theology distinguishes: (1) evangelism (cross-cultural or otherwise, preaching the gospel to any unbeliever); (2) missions in the narrower sense (taking the gospel to unreached people groups, those with no access to the gospel); (3) disciple-making (the full work of raising up mature followers of Christ).
The missions mandate is not optional or peripheral; it is constitutive of the Church. Five observations. (1) God is a missionary God. "God so loved the world that He gave His only Son" (John 3:16). The Father sent the Son; the Son sends the Church. This is missio Dei — the mission of God that precedes and undergirds the mission of the Church. (2) The scope is the nations. Revelation 5:9 shows the Lamb purchasing "for God with His blood people from every tribe and language and people and nation." The purchase is not hypothetical — it is effective. Somewhere in every tribe, language, and nation the Lamb has purchased a people who will certainly be saved. Missionaries go to find the purchased, not to hope that people respond. (3) The unfinished task. About 3,200 people groups remain "unreached" — less than 2% Christian, with no viable indigenous church. These groups include about 3 billion people. The missionary task is more finishable now than ever (Bible translation, aviation, internet, mobilization networks), yet most Western Christians give and pray as if unreached peoples did not exist. (4) Supporting the missionary. 3 John 5-8 — "it was for the sake of the name that they have gone out, taking nothing from the Gentiles. Therefore we ought to support people like these, that we may be fellow workers for the truth." Giving, praying, hosting, training — every Christian who does not personally go is called to enable those who do. (5) Martyrdom is normal. More Christians have died for Christ in the 21st century than in any previous century; serious missions has always cost lives. The missionary vocation is not "safe career with spiritual upside"; it is carrying the cross into harm for the sake of the Name.