What is the main theme of the first six chapters of 2 Corinthians?
The opening six chapters center on Paul defending his apostleship. Because Judaizers challenged his authority and insisted Gentiles also keep the law, Paul recounts the suffering he endured so Gentiles could receive the message of grace. This defense dominates the section, showing the Corinthians what he underwent for the gospel.
Les explains that as Paul went among Gentile cities preaching the gospel, Judaizers kept attacking him, saying he never walked with Jesus like Peter did and was just an upstart. Under those attacks, Paul had to defend his apostleship repeatedly, referring over and over to his sufferings so that Gentiles could get the message of grace. Les says this is basically the whole theme of the first six chapters: Paul again defending his apostleship and showing the Corinthians all he had gone through for the sake of the gospel.