As Jesus taught, forgiveness is “from your heart” (Matthew 18:35). Compulsory forgiveness is cold and false. Forgiveness might need some prodding from time to time, but, in the end, it should be voluntary and free.
During Paul’s imprisonment in Rome (Philemon 1; cf. Acts 28:30–31), a household slave named Onesimus stole from his master Philemon and ran away (cf. Philemon 18–19). If Philemon caught Onesimus during this time, he could have severely punished him as a result. Philemon’s wife Apphia and his son Archippus would have known about the situation, and the church that met in their house was likely aware as well (cf. Philemon 2). Onesimus’s sins affected many.
As time went on, Onesimus somehow found Paul in Rome, and Paul led him to saving faith in Christ (cf. Philemon 10). As much as Paul wanted to keep Onesimus with him, he returned him to Philemon whose say determined Onesimus’s future (Philemon 11–14). Paul encouraged Philemon to receive Onesimus as a brother in the Lord, and Paul took Onesimus’s debts upon himself (Philemon 15–19).
Paul assumed in advance what we can assume in retrospect today—Philemon forgave Onesimus. Paul was confident of this forgiveness, and Philemon may have gone beyond Paul’s words to free Onesimus from servanthood as well (Philemon 21). In fact, Paul was so confident of the matter (and his release from prison) that he asked Philemon to prepare a room for him to use in a future visit (Philemon 22).