What impact did the Roman Empire have on the spread of Christianity?

Without the Roman Empire, Christianity might never have spread to become the dominant religion of the West. The Roman Empire facilitated safe and efficient travel, thus allowing the apostles to spread out from Palestine into neighbouring countries and, eventually, throughout the entire Roman Empire.
Early in the fourth century, Emperor Constantine gave Christianity state patronage, thus encouraging the rapid spread of the new religion. By the end of the fourth century, Christianity was the state religion of the Empire and public worship of the pagan gods was banned. Church leaders drove paganism underground, persecuting anyone found to be worshipping the old gods. With imperial support, the Christian Church also expropriated all the pagan temples, land and wealth. The now wealthy Church was in a sound position to extend its control over the people of the entire Roman Empire.