Onesimus, also called Onesimus of Byzantium and The Holy Apostle Onesimus in the Eastern Orthodox Church, was a slave to Philemon of Colossae, a man of Christian faith. He may also be the same Onesimus named by Ignatius of Antioch as bishop in Ephesus which would put Onesimus's death closer to 95. If so, Onesimus went from slave to brother to bishop.

The Epistle to Philemon is one of the books of the Christian New Testament. It is a prison letter, authored by Paul the Apostle, to Philemon, a leader in the Colossian church. It deals with the themes of forgiveness and reconciliation. Paul does not identify himself as an apostle with authority, but as "a prisoner of Jesus Christ", calling Timothy "our brother", and addressing Philemon as "fellow labourer" and "brother". Onesimus, a slave that had departed from his master Philemon, was returning with this epistle wherein Paul asked Philemon to receive him as a "brother beloved".