In the early days of Christianity travel and communication were arduous at best, but the Romans did have a road system. Since the Holy Land was at that time part of the Roman Empire it was possible to travel into southern Europe to Rome, with cities in Greece and what is now Turkey along the way. As the Apostles and other disciples of Jesus traveled they were able to evangelize in major population centers to a certain degree. Remember, since Julius Caesar the Roman Emperors declared themselves to be gods, and were threatened by a monotheistic religion.
Secrecy in the early Christian communities was essential, since the Roman Emperors made being a Christian illegal.