How could Paul say he was not ashamed despite all his suffering?
Paul was not ashamed because he knew whom he had believed. In spite of all his difficulties, sickness, sorrow, and suffering, he never once doubted the God whom he served. His sufferings came from opposition and satanic resistance to the gospel going to the Gentiles, but his trust in God held firm.
Les explains that Paul's suffering came not only from Jewish opposition—both believing Jews and those still steeped in Judaism—but also from satanic power from every direction. Satan did not want the gospel going to the Gentile world; it had gone to Israel throughout the Old Testament, but now to the masses of Gentiles was the last thing Satan wanted. So Paul suffered for these things. Nevertheless, he says, “I am not ashamed. For I know whom I have believed.” Les emphasizes that in spite of all Paul's difficulties, sickness, sorrow, and suffering, he never once doubted the God whom he served.
For the which cause I also suffer these things: nevertheless I am not ashamed: for I know whom I have believed, and am persuaded that he is able to keep that which I have committed unto him against that day.