The mystery (great spiritual insight) that was revealed to the apostle Paul is this: that Jesus came as an atonement for the sins of all men, the ushering in of the new covenant that is by grace and not by human effort, that we can become heirs to the promise not by physical lineage but by spiritual rebirth. This mystery was revealed to him by the Holy Spirit, bringing together the message of the Old Testament and of Jesus’ teachings.
This revelation came with a mission: he simply cannot just keep this great news to himself. In spite of troubles, he is able to persevere and be joyful. His prayer is for us to know and understand the “breadth, length, height and depth” of God’s love shown through Jesus.