After Paul had left Colossae, and was in prison (probably at Rome), Onesimus ran away, taking some of his master's money, and eventually arrived in Rome, where he met Paul, and became a Christian. Paul sent him back to his master, but with a covering letter. Paul indeed sent Onesimus (whose name means "useful") back as a slave - as he was required to do in Roman law (after all, Onesimus was someone else's property, not his to set free) - but with a brilliant rhetorical move, he left Philemon with no honourable option but to set him free.