For a thousand years from the days of the apostles, Christianity was a working missionary institution.
In the first four centuries of its history the church won the Roman Empire from heathenism to Christianity. Afterward, its missionaries met the advancing hordes of barbarians and conquered them before they conquered the Western Empire. After the tenth century, church and state, pope and emperor, were in strife for supreme control, and the missionary spirit declined, though it was never entirely lost. The Reformation was concerned with efforts to purify and reorganize the church, rather than to extend it.
We have seen that in the later age of the Reformation the first extensive effort to Christianize the heathen world was made, not by Protestants, but by Roman Catholics under St. Francis Xavier.