St. Paul was an apostle who vigorously spread Christianity throughout the known world at the time. He went on three missionary journeys, establishing Christian communities across places like Cyprus, Asia Minor, Greece and Rome. Some of the key communities he founded and visited included those in Philippi, where he established one of the first in Europe, Thessalonica, Athens, and Ephesus, where he stayed for three years. Through his missionary work and letters, St. Paul made major contributions to the growth and organization of the early Christian church.