St. Paul Not called Apostle in His Own Time

Surprisingly enough, St. Paul was not considered an apostle by most people in the early Church. This is unthinkable today, but it was St. Paul's reality, which at times was a tremendous challenge for him. In 2 Corinthians, Paul painfully expresses his sorrow when the Corinthians reject him because they agree with Paul's critics that he is not an apostle. How did St. Paul defend his apostleship and win them back as his beloved community and spiritual children?