The heart of missions is about God glorifying Himself in the world through the gospel. Through missional efforts to make Jesus Christ and His gospel known in the world (both locally and globally), we join with God as He glorifies Himself through the gospel, saving those He wills who formerly united in rebllion against Him.
This salvation is through faith in Jesus Christ and His voluntary substitutionary atoning death as the only possible restitution for man’s rebellious sins. Salvation is entirely of grace and not of works since man brings nothing but sin to the equation.
Faith comes by hearing and hearing by the word of Christ. Since these things are true, it is the duty of all disciples of Christ to make the gospel known.