Why was Paul in prison?
Paul was in prison because of his apostleship. He had been saved on the road to Damascus and sent to the Gentiles with a new revelation, not based on Judaism or the Law. As he preached the Gospel of grace, he turned the Roman Empire upside down, and his apostolic mission led to imprisonment.
Les explains that Paul hated everything associated with the Christ of Calvary and thought he was destroying his pet religion, Judaism. But God in grace saved him on the road to Damascus and immediately gave instructions that he would go to the Gentiles with a whole new format and new truths never before revealed. Paul was constantly confronted with the abject immorality and gross pagan wickedness of his day, yet by simply preaching the Gospel of the grace of God he turned the Roman Empire upside down. As he ends up in prison because of his apostleship, he writes his next three letters to the saints, the believers.