Where was the ministry of the Twelve confined?
The ministry of the Twelve was confined to the nation of Israel. They did not carry the gospel into the broader Mediterranean world or Gentile territories, so Paul's apostolic work in places like Corinth, Athens, Ephesus, and Asia Minor was virgin territory rather than building on their foundation.
Les explains that the Twelve seldom went beyond the Jordan River and had no ministry outside of the nation of Israel. Because they did not cover the whole area of the Mediterranean, Paul could not be accused of boasting in another man's line of things made ready to his hand. Instead, Paul hoped to preach the gospel in the regions beyond Corinth—Rome, Spain, Great Britain, and eventually the New World—where the Twelve had not gone. Every place Paul went was virgin territory because their ministry remained confined to Israel.