The Roman Empire
After their return from Babylon to Jerusalem, the history of the Jews for the next 400 years was troubled by wars. Then, about 60 years before Jesus was born in Bethlehem, the Romans, who came from Italy, conquered Palestine. The Roman Empire was the greatest there had ever been, stretching as it did from the English Channel to North Africa and from Spain to Palestine itself.
Under the Romans the world enjoyed a long time of peace such as it had never known before, and that meant that one could travel all over the Empire in safety. A few years earlier the Mediterranean Sea had been very dangerous on account of pirates, but now they had all been rounded up by the same general who captured Jerusalem. It was also safe and easy to travel by land because the Romans made wide, first class roads which went from Rome, the capital, to all parts of the Empire. One of the best known in Britain was Watling Street which ran from Canterbury to Chester. Many of these roads have been resurfaced and parts are still in use today.
As a result, one could get without difficulty from Palestine to Spain either by land or sea. When the Roman Empire fell to pieces, travel never became as easy again until the invention of railways in the middle of the 19th century.
In those days there were no such things as passports because the world, being under one government, was more like one big country. A Roman citizen might be an Italian or a Jew or an Egyptian.
For these reasons St Paul and the early Christian missionaries were able to go where they wanted without any difficulty, which meant that the Church could spread rapidly all over the Empire.
Another thing which makes travelling abroad today not too easy is the difference in language. Most countries have their own language which means that, if you wanted to teach these people you would first have to learn French, German, Polish and so on.
In the time of Jesus, however, there was one language which almost everyone understood and that was Greek. No matter where you went or to whom you spoke, you would be all right if you spoke Greek, just as in North America today everyone speaks English. That is why St Paul and the other Apostles had no difficulty in making themselves understood. Actually they spoke two languages, Greek and also Aramaic, the language of Palestine, just as in Wales today people speak both English and Welsh.