Overview: How did a small group following an apocalyptic prophet in Palestine become Christianity - what is now called a "world religion"? This small movement saw many changes in the second, third, and fourth centuries, from the development of different sects, philosophical theologies, and martyrology, to the rise of monasticism, and finally to the ascension of Constantine to the throne and the Christian Roman Empire. It was not until the nineteenth century, however, that the term "world religion" came to be used and Christianity was categorized as such.