Final answer:
The spread of Christianity primarily occurred from AD 400-699, starting as a distinct religion in the first century. It became the dominant faith in the Roman Empire during the reign of Emperor Constantine and shaped global timekeeping conventions.
Step-by-step explanation:
The spread of Christianity occurred primarily from the first through the sixth centuries, which corresponds to the time frame AD 400-699. Christianity emerged as a distinct religion in the second half of the first century and was spread throughout the Mediterranean world, influencing societies and empires profoundly. Under the reign of Emperor Constantine from 306-337 AD, Christianity began to transition to the dominant religion of the Roman Empire, marked by Constantine's adoption of the faith and the subsequent Theodosian edict making it the state religion.
Christianity's expansion was facilitated by various means, including the travels of Christian apostles like Saint Mark to Alexandria in the middle of the first century, leading to the establishment of significant Christian communities. The adoption and spread of Christianity were further propelled by the later Age of Exploration, leading to its spread beyond the borders of the Old World. Moreover, religious calendars such as Anno Domino (AD) and Before Christ (BC), developed by Christian monks like Dionysius Exiguus in the 6th century, reflect the influence of Christianity on global timekeeping practices.