Tom Steller, who wrote the afterword to Let the Nations be Glad, answers,
A world Christian is someone who is so gripped by the glory of God and the glory of his global purpose that he chooses to align himself with God's mission to fill the earth with the knowledge of his glory as the waters cover the sea (Hab 2:14). Everything a world Christian does is with a view to the hallowing of God's name and the coming of God's kingdom among all the peoples of the earth. The burning prayer of the world Christian is, "Let the peoples praise you, O God; let all the peoples praise you!" (Ps 67:3).
So whether we are those who send or those who go, let us glory in the supremacy of God in missions, and let us link arms together as we join the refrain of old, "Let the nations be glad!"
Stellar also reflects on how missionaries are treated in the churches.
The reason we must send them in a manner worthy of God is that they go out for the sake of the name.
The name of God is at stake in how we treat our missionaries. God is glorified when we support them substantially with our prayers, our money, our time, and myriad other practical ways.
God is not glorified when our missionaries are simply a name on the back of the church bulletin or a line item in the budget.