Because it originated as a Jewish sect in a Roman province of Asia Minor and without a firm spiritual anchorage in the beginning, Christianity became almost from the very start a missionary religion.
The written fundaments were developed in the course of many centuries, starting with the writings of the apostles and evangelists. First, Christianity conquered the eastern provinces of the Roman empire.
It was believed that God had only created the Roman empire in order to provide the conditions for the expansion of Christianity. The administrative organization, the wide spread use of Latin and Greek languages, the relatively well developed means of communications by sea and land and, above all, the cosmopolitan and multi religious Roman world paved the way for this new religion indeed, though its success was for from sure.
The religion prevailed however, but in first instance in the Middle East and not in Europe. When one considers that Christianity is in origin a Middle East religion, its close relations with and in Europe is one of the paradoxes of history.
The other paradox is that Christianity has a relative small impact on present-day societies and cultures in the Middle East. The rise of Islam and the spilt of the Church into Byzantine, Christian and numerous local Christian religions contributed to this small impact.
There are many other reasons for the relative failure of Christianity to have a bigger influence, the fact remains that the territorial roots of Christianity are about to vanquish in large parts of the Middle East. Christianity and Islam were from the very beginning not only religions, but also social and cultural movements.
Both religions have common roots in Judaism, share the same religious figures and sites and are often based on the same stories, legends, dreams or writings.
The three religions originate from the same region and share the same fragmentation. By the last decades of the sixth century, just before the rise of Islam, Christianity was divided in sentiment, language, cult and organization.
The Monophysites dominated a huge area extending from Armenia in the north to Ethiopia in the south and including much of Syria, Mesopotamia and the Persian frontier, Chalcedonian orthodoxy prevailed in Palestine, Asia Minor and the Balkan provinces of the Byzantine empire, Coptians lived in Egypt.
Nestorians and Arians, actually, there were many Christian worlds and the Roman papacy and Christianity was weak and in an early stage of development. Even in today’s heartland of Christianity, Europe, the differentiation between eastern (Greek) and western (Latin) was obvious.
The extremely fast expansion of Islam was mainly due to the fragmentation of Christianity and the vacuum left by the fall of the Roman empire. After initial successes, Islam shared the fate of Christianity however and differentiation became the splitting force.
Almost fifteen hundred years after the rise of Islam, it is still too early to judge the future development of this religion, like nobody predicted the developments in the Christian Church of the past fifty years. G. Barraclough, The Christian World. A Social and Cultural History of Christianity (London 2003).