Life and Confession
Agathonicus lived in Nicomedia (modern İzmit in present-day Turkey), then a major city of the Roman Empire in Asia Minor. According to the tradition he came of an illustrious family and was well versed in Scripture, which he used to teach and to convert many from paganism to the Christian faith. He is described as a prominent figure, the OCA life calling him a leader within the Senate.
The Wikipedia account adds that while living in Greece he taught the Greeks about Christianity, before the persecution overtook him and his companions.