Ehrman gives an excellent summary of Paul’s letters in the book Peter, Paul, and Mary Magdalene:

In his letter to the Romans Paul has to defend himself against charges leveled by other, trusted Christian authorities that he preached a gospel that leads to lawless behavior and that impugned God on the ground that he reneged on his promises to the Jews. In 1 Corinthians he attacks those who think—based on his own teaching, evidently—that they have already experienced the glories of a spiritual resurrection with Christ. In 2 Corinthians he attacks new apostles who have arrived in town with a similar message, sarcastically calling them “super-apostles.” In his letter to the Philippians he attacks Christians who urge his converts to be circumcised, calling them “dogs” who were concerned only for their own pleasure and who “worship the belly.” And most notably in Galatians, he fires off a letter of reproach for Jewish Christians of similar ilk, who have argued that the covenant God made with the Jews was eternally binding. For Paul, these so-called Judaizers stand under God’s curse, and anyone who accepts their message has “fallen from grace.”