St. Onesimus was a slave to Philemon, an influential man who had been converted by St. Paul. Onesimus offended Philemon and fled in order to escape any sort of retribution. He then met St. Paul while Paul was in a Roman prison. Shortly after, Onesimus was baptized.
Paul then sent a letter to Philemon asking for Onesimus' freedom, so Onesimus could become one of his own assistants. This letter is the Epistle to Philemon and entreats Philemon to accept Onesimus “no longer as a slave, but more than a slave, a brother, beloved especially to me.”
Philemon pardoned Onesimus and he returned to faithfully serve St. Paul. We know that St. Paul made him, with Tychicus, the bearer of his Epistle to the Colossians. (Col. 4:7-9)
Later, as St. Jerome and other fathers testify, he became an ardent preacher of the Gospel and succeeded St. Timothy as bishop of Ephesus.
He was cruelly tortured in Rome, for 18 days, by a governor who was infuriated by his preaching on the merit of celibacy. Onesimus' legs and thighs were broken with bludgeons before he was stoned to death.
His martyrdom occurred under Domitian in the year 90.