It is impossible to understand the spread of Christianity without a good understanding of the Roman empire which had reached its peak under the emperor Trajan in AD 98-117. Christianity started in the small Roman province of Judea which would have been seen as a small outpost of a much larger empire that stretched from Iberia and Mauritania in the west, Armenia and Mesopotamia in the east, Britannia and Germany in the north and North Africa in and around the area of Egypt. It covered most of modern day Europe, the middle east, the Mediterranean area and western Asia. Rome began its expansion as a republic but because of political weaknesses in this system it soon became governed by a central figure of Authority, an Emperor. (Emperor meaning one who gives orders) The term King was not used due to previous abuses of power in the republican era.
It was during the reign of Caesar Augustus that Lukes gospel places the birth of Jesus and during his reign the Roman empire continues to expand. Egypt becomes of particular importance in order to supply grain to the Roman population. Tiberius who succeeded Augustus reigning from AD 14-37 was ineffective and spent long periods of time in seclusion on the Island of Capri. Later under Trajan the stability and expansion of the empire was restored including a major programme of public building which took place in Rome and which epitomized the empires status as the greatest that the world had ever known.
A form of civil religion began to emerge at that time which was linked with worship of the Emperor, Roman state and empire who and which was worthy of ultimate honour, like a type of state divinity. Refusal to worship the Emperor and follow him was seen as an act of treason which proved very problematic for the early Christians who worshiped the God of Jesus Christ.
The administrative and commercial links that were established by the Roman empire made it very easy for new ideas to spread. A common language (Latin) helped and although local languages were allowed it was latin that was predominant and which continued right through the middle ages and was the tongue through which the Christian faith was communicated and spread. Transport was easy as there were crossroads of trade routes centred in Judea going north south east and west. These routes were also used by the army when moving across the empire. Sea travel was also relatively safe as the Roman navy suppressed piracy in the region. As well as for military use the trade routes were used by immigrant workers travelling to and from the Roman colonies. People brought their religious and cultural traditions with them but all these had to be subservient to the imperial cult.
The word religion means to 'bind together' but was also understood as devotion (pietas) so religion was seen as a social attitude and activity that promoted unity and loyalty to the state. Roman citizens were allowed to adopt other religions and beliefs in private but these were not to conflict with the official state religion in public. Many of these cults were brought back from Greece and Asia by Roman soldiers and merchants. Christianity could easily have fitted into the private sphere but for the fact that Christians were commanded by Jesus to proclaim the gospel in public. Christianity was therefore in conflict with the Roman authorities from the outset.
Summary and references from Christian history-An Introduction by Alister E McGrath