John Bunyan preaches about the concept of unconditional pardon from God, illustrating it through the story of a man who received a royal pardon despite his treason. He emphasizes that just as the man should honor his king by believing in the pardon, believers should trust in God's mercy and the sacrifice of Christ, rather than being troubled by their past sins. Bunyan reassures that despite the accusations from the devil, the law, and one's own conscience, the promise of eternal life through Jesus remains steadfast. He encourages the congregation to embrace this divine pardon and not be swayed by reminders of their former lives.