Onesimus is venerated among the Seventy Apostles and is best known as the subject of the Apostle Paul's Epistle to Philemon. By the account preserved in the synaxarion, he was a slave in the household of Philemon, a Christian of Colossae in Phrygia. Having committed an offense against his master and fearing punishment, he fled to Rome, where as a runaway he was imprisoned. There he encountered the Apostle Paul, who instructed him in the faith and baptized him.
Paul, who had grown to love Onesimus during his imprisonment, sent him back to Philemon with a letter asking that the runaway be forgiven and received no longer as a slave but as a brother. This short letter is preserved in the New Testament as the Epistle to Philemon. According to the tradition, Philemon forgave Onesimus and sent him back to serve the Apostle Paul.
After Paul's death, Onesimus continued to serve the apostles and in time was ordained a bishop. The synaxarion relates that he preached the Gospel in many lands and cities, naming Spain, Carpetania, Colossae, and Patras, and that in his old age he occupied the episcopal throne of Ephesus, succeeding the Apostle Timothy. He was arrested under the emperor Trajan and put to death for the faith around the year 109, and is commemorated by the Church on February 15.