Onesimus was a servant of Philemon, who was a man of love and treated his servants with kindness. He was shown to be a bad servant, by taking advantage of his master's kindness, stealing him, and escaping from Colloseis. He went to Rome, where he was catechised into the Christian faith by apostle Paul, was baptised, and became a man wonderful in virtue. From a worthless slave he became an apostle of Christ, really useful and truly free.
Apostle Paul, was imprisoned in Rome due to the Gospel, and certainly wanted to keep Onesimus close to him, but did not want to do so without the favourable opinion of Philemon. That is why he sent him back Onesimus with a reference. This is the letter to Philemon, an excelent text, now a part of the New Testament. In his letter, he certifies Philemon about the spiritual renewal of his servant and ask him to receive him, no longer as a slave, but as a beloved brother. Apostle Philemon accepted him with joy, but sent him back to Rome in order to serve apostle Paul.
After the martyrdom of Paul, Onesimus was also caught, and in the name of the Gospel suffered horrible tortures. Finally, they broke his bones, and thus he left this ephemeral life and went to the eternal one.
His holy memory is kept on the 15th of February.