"Every heart with Christ, a missionary; every heart without Christ, a mission field."
- Dick Hillis, former American Protestant Christian missionary to China
Here is an awesome study of modern missions: "Salvation to the Ends of the Earth" by Andreas J. Köstenberger and Peter T. O'Brien. I've only read about 1/3, but seriously recommend it to anyone interested in biblically rigorous theology on what God's Word says from Genesis to Revelation on the topic of mission.
Here is an introduction to Monergism.com's book review:
In light of the overwhelming abundance of materials on the practical aspects and New Testament examples of Christian mission, the relative scarcity of works intending to set forth the foundational principles of mission from the entire gamut of scriptural witness is conspicuous. This self-styled “biblical theology of mission” admirably fulfills a much-needed role in the pursuit of a rigorously biblical and redemptive-historically comprehensive framework for modern mission. more...
From God's Word: “he says: ‘It is too small a thing for you to be my servant to restore the tribes of Jacob and bring back those of Israel I have kept. I will also make you a light for the Gentiles, that you may bring my salvation to the ends of the earth.’” – Isaiah 49:6