How did political and economic conditions in the roman empire aid in the spread of christianity as the roman empire declined?

The political unity of the Roman Empire did produce a certain economic and political stability, notwithstanding its many faults. This encouraged trade between large cities and regions
2.    The military and trade routes meant relatively easy access to large numbers of people (both by land and sea). Joel Kotkin writes, ‘Rome allowed considerable self-government to individual cities; the empire itself, notes the historian Robert Lopez, functioned as a ‘confederation of urban cells.’ Europe would not again see such a proliferation of secure, and well-peopled cities until well into the nineteenth century. People, products, and ideas traveled quickly through the vast archipelago of ‘urban cells’ over secure sea-lanes and fifty-one thousand miles of paved roads stretching from Jerusalem to Boulogne…Christianity’s rapid growth could not have taken plave without the empire’s expansive urban infrastructure.’ [i]
3.    The universal use of Greek as a result of former conquests aided communication
4.    The cosmopolitan atmosphere of the Empire – mixed cultures – enabled easier cross-cultural evangelism e.g., Jews who were culturally Greek (Barnabus from Cyprus, Paul the Roman citizen) were able to bridge cultures
5.    The very real and lasting impact of the ministry of Christ and the earliest apostles

To cite one illustration of the impact of the many miracles that Jesus and the early Christians performed,  Quadratus, writing very early in the second century, says:

“Our Saviour’s works were always there to see, for they were true – the people who had been cured and those raised from the dead, who had not merely been seen at the moment when they were cured or raised, but were always there to see, not only when the Saviour was among us, but for a long time after His departure; in fact, some of them survived right up to my own time.” [ii]

Those healed in the gospel accounts, as well as others, continued to be witnesses of Christ’s power for many years. The spread of the gospel through the Empire was assisted by these various factors, but the central message was consistent, and centred on Jesus Christ, His compassion, His power and His Lordship.