Christianity was once so localized to Europe that the term “Christendom” largely meant Europe. After the fall of Constantinople in 1453, the last outpost of Christian rule in the East was lost. The history of Medieval Europe was flecked with Christians holding back Islam. The Battle of Tours in France prevented the overrunning of Western Europe by Muslims. The Reconquista in 1492 ended the remnants of Muslim rule in Iberia. The Battle of Lepanto ended the threat of Muslim naval power in the Mediterranean.

Having captured the Roman world from within (following the Edict of Milan signed by the emperors Constantine I and Licinius in 313 that proclaimed religious freedom in the Roman Empire), largely because of its enormous sympathy for women, children, and slaves, Christendom gradually spread out of Europe into the other parts of the globe. Indeed, an overt purpose of much early exploration was to bring the Gospel to the peoples outside Europe or the Middle East (where it was familiar).