In relation to Cenchrea, Corinth was located on the opposite side of the isthmus that connected the Peloponnesus to the mainland of Greece. Ironically, although the city was ancient in the first century A.D., it was quite new.
In 146 B.C., the Romans burned Corinth to the ground, killed all the city's males and sold both women and children into slavery. A century later, Julius Caesar completely rebuilt the city. It subsequently grew so rapidly that it quickly became an important economic area in the Empire and the seat of government for the province of Achaia (see Acts 18:12 - 16).
Corinth, along with Jerusalem, Syrian Antioch and Ephesus, was one of the few prominent centers of early Christianity. It also one of the few cities Paul stayed in for an extended period (the others being Caesarea and Rome (as a prisoner), Syrian Antioch, Ephesus and Tarsus).
Corinth boasted several house churches. A group of Christians met in the home of Justus, whose residence was right next to a local Jewish synagogue (Acts 18:7). Those who also hosted brethren in their home were Chloe (1Corinthians 1:11) and Gaius (Romans 16:23), who was one of only a very few people personally baptized by Paul (1Corinthians 1:14). A home fellowship may have also been hosted by a man named Crispus (1Corinthians 1:14).
Philippi, anciently, was the site of a rich gold mine. King Philip II of Macedon (the father of Alexander the Great) seized the mines in the area, fortified the city, and named it after himself. Later Philippi became a Roman colony (Acts 16:12) and, at the time of the New Testament, was more a military town than a hub of major commerce.
Paul visited Philippi on his second and third evangelistic journeys. It is the place where he made his first convert on the European continent, a woman named Lydia, who traveled extensively due to her business of selling purple dyed fabrics (Acts 16:13 - 15). It is also the location where the first European Christian church started.
The believers in Philippi and the surrounding area were special to Paul, as they were the only ones who supported him financially, even as he ministered in other areas (2Corinthians 11:7 - 9, Philippians 4:15 - 18).
Thessalonica was Macedonia's most populous city, maintained the province's largest port, and was its capital city. It was founded in 315 B.C. by Cassander, a relative of Alexander the Great who also was one of his generals. It was located on the Thermaic Gulf in Greece and at the end of a major trade route that started at the Danube.
Thessalonica (along with Corinth) was the two most important economic centers of all of Greece. As a designated "free city" in the Roman Empire, Thessalonica had no army garrison within its walls and benefitted from the privilege of striking its own coins.
The Apostle Paul evangelized the northern Greece city during his second and third journeys. A man named Jason, a Jewish Christian who was likely a blood relative of Paul (Romans 16:21), hosted believers in his home at Thessalonica (Acts 17:5 - 9). The books of 1 and 2Thessalonians, penned by Paul from Corinth between 50 - 51 A.D., are noteworthy in that they are the first ones written by the apostle that are included in the Bible.