An interesting insight into the way Christianity has spread throughout the world is given by J Herbert Kane’s classic book A Concise History of the Christian World Mission: A Panoramic View of Missions from Pentecost to the Present. The book is a reasonably brief and very readable introductory survey of the topic. It starts with Christianity in the Roman Empire and ends with a discussion of the future of missions work.
Along the way, the book describes a number of interesting things. During the first two centuries, a common theme was the fortitude and love with which Christians withstood persecution, so that the faith spread through the blood of the martyrs. After the emperor Constantine became a Christian and Christianity became a State religion, the faith was often spread by force rather than through persecution.
An extraordinary period of Roman Catholic global evangelism occurred between 1300 and 1700AD. A mission in China had 100,000 converts by 1330, but the faith was wiped out by persecution some 40 years later. Francis Xavier had a fruitful mission to Japan in the 16th Century with 150,000 converts. Catholic missions to the Congo started in 1491, 350 years before David Livingstone.
The book is now almost 30 years old, so the “current” information is now quite dated, but it is still well worth reading.