A small fragment of papyrus – which was used throughout the ancient Mediterranean world for writing or painting on – was found dating back to the 230s A.D.
It's thought to be older than all the other Christian documentary evidence of Roman Egypt.
The letter originates from the village of Theadelphia in central Egypt and was sent by Arrianus to his brother, Paulinus.
"Greetings, my lord, my incomparable brother Paulus," the letter reads.
"I, Arrianus, salute you, praying that all is as well as possible in your life."
The letter includes updates on the family and even a simple request for some fish liver sauce.
The document is part of a collection of ancient manuscripts at the University of Basel in Switzerland.
The letter provides valuable insights into the world of the first Christians in the Roman Empire, which is not recorded in any other historical source.
The earliest Christians in the Roman Empire are usually portrayed as eccentrics who withdrew from the world and were threatened by persecution, the researchers explained in a press release.
The letter also sheds light on the brother's faith, with Arrianus wishing his brother well "in the Lord".
"I pray that you fare well in the Lord," he adds.
According to University of Basel researcher Professor Sabine Huebner, the brothers appear to have been young, educated sons of the local elite, as well as landowners and public officials.
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