Judgment Day
The apostle Paul was one of God's most faithful servants. And I believe there were three important motivations behind Paul's faithfulness: hope, love and fear.
Paul had a blessed hope of eternal life that motivated him to faithfulness. He also had a great love for Christ. In 2 Corinthians 5:14 he says, "The love of Christ constraineth us"— meaning, it constrained him to remain faithful to God.
But Paul's faithfulness was also motivated by something else: the reverential fear of the hour he would stand before the Judge of the world on Judgment Day!