As far as we know, all of Paul's travels were within the Roman Empire. The language of commerce and culture would have been either Latin (in the west) or Greek (in the east). The basic structures of law, government, and commerce would have similar throughout. The Roman roads and Roman ships would have made travel relatively easy throughout most of the Empire.
However, the local customs would differ greatly from one place to the other. Also, the languages spoken in the homes of the people would often be a local language. The people to whom Paul ministered would represent the full range of racial distinctions. The large cities of the day (like Antioch and Rome) were very cosmopolitan. Cultures and ethnic groups mixed freely.
Most of the people would have been polytheistic, believing in many gods. Their practices would have differed from place to place, but they would have been united in the worship of the Roman Emperor. In fact, Emperor worship was considered the religious glue that helped hold the Empire together. The Romans did not care who people worshipped as long as they also worshipped the Emperor. This explains why the early Christians were considered subversives. When they turned to Christ, they did not recognize any other God. In fact, one of the early accusations against them was that they were atheists--because they did not believe in the gods.
