In his commentary on 2 Corinthians (NIVAC), Scott Hafemann sees Paul’s apologetic for his ministry implicit throughout the letter, starting at the beginning (2 Cor. 1:3-7). I heartily agree. He notes that Paul opens his letter with thanksgiving to God for the very thing which his opponents are using to call his ministry into question in the minds of the Corinthians: his manifold sufferings (61). As one works through 2 Corinthians, it becomes obvious that Paul considers his sufferings as an apostle to be one of the proofs that he is truly qualified as an apostle. This is directly opposed to the assertions of the false apostles that are plaguing the Corinthian church, who would hold up Paul's sufferings as proof that he is not a true apostle.
Paul understood suffering for the sake of Christ as a central part of his calling as an apostle right from the beginning of his Christian experience. In Paul's Damascus Road vision of Christ, his conversion, and in his commissioning three days later, Paul understood that suffering would be a sign of his apostleship (Acts 9:3-19). In Paul's robust theology of union with Christ in his death, burial and resurrection (Rom. 6:3-11; Gal. 2:20), we can see that Paul saw the story of Christ's salvation-accomplishing event in the story of his own Damascus salvation event. In the Damascus event, in the vision of Christ and the words he spoke, Saul died to self and to his old perceptions and ways of understanding and relating to God (Phil. 3:4-11), and in three days he was raised to resurrection life, when he received the Spirit, his eyes were opened, his gospel mission was explained to him, and he was baptized. In fact the way Paul speaks of baptism as a dying and being raised to new life in Christ shows us just how profoundly his own experience, interpreted though the light of the Spirit, shaped his theology (Rom. 6:4; Col. 2:12).
Paul was promised that suffering would be a part of his apostolic calling, his gospel mission. As Jesus said to Ananias when he was preparing him to meet Paul,
“Go, for he is a chosen instrument of mine to carry my name before the Gentiles and kings and the children of Israel. For I will show him how much he must suffer for the sake of my name.”
(Acts 9:15-16 ESV)
So long as Paul continued to speak the true gospel and suffered for it, he could rejoice. Why? Because it was a confirmation that he was fulfilling his mission. God was showing Paul how much he would have to suffer for Christ's sake. And because it was for Christ's sake that he was suffering, Paul rejoiced. In his second letter to the Corinthian church, Paul pointed to his sufferings as a sign of his apostolic legitimacy in the face of challenges from the false apostles. Anyone who had heard the story of Paul's conversion-commissioning would know that Christ had not only called him to take the gospel to Gentiles, kings and the children of Israel, but they would also know that Paul was promised suffering for the sake of the gospel as well. Paul could thank God for his suffering because he saw it as confirmation that he was truly fulfilling his apostolic gospel mission in the way God had told him from the start that he would.