The Catechism in Questions & Answers

391. How did the church of Jesus Christ develop?

After Pentecost, when the Holy Spirit was poured out upon the believers, the church of Jesus Christ continued to develop: Apostles and other ministers began to work within it. The gospel was preached and the sacraments were dispensed.
Congregations came into being all throughout the Roman Empire. Christianity began to spread among both Jews and Gentiles.

At the time of the early Christians, the Roman Empire was the power that ruled the world. It encompassed the entire Mediterranean region as far as the Middle East. The Roman Empire, with its good transportation networks and uniform common language of Greek (and later Latin), was of great advantage for the spread of the gospel.