The Apostle Paul again defends himself and his apostolic authority by detailing his suffering on behalf of all the churches he served.  To him, his sufferings authenticated his credentials as an apostle; they qualified him for this role, not disqualified him as his critics charged.  Halfway through this passage, Paul warns those in the Corinthian church who supported him to separate from those who stood opposed to him.  He addresses them as unbelievers and declares that it was impossible to be in fellowship with them.  If the believers are to open their hearts to Paul, they will have to disassociate from those within the church who reject Paul and his ministry.