Onesimus ran away from his master refers to Philemon. This is a reference to a letter written by Paul to Philemon, a Christian in Colossae who owned a slave named Onesimus. Onesimus had run away from Philemon and somehow ended up with Paul, who converted him to Christianity. In the letter, Paul urges Philemon to forgive Onesimus and accept him back not as a slave, but as a brother in Christ.