Thank you for the question, “Who wrote the first dictated letter?”
If one is referring to the New Testament, it is most likely Caesar Augustus when he issued a decree that a census should be taken of the Roman empire (Luke 2:1). It is noted that rulers would often dictate their decrees in a letter form to be sent out to his kingdom.
The next noted letter is when Saul of Tarsus received letters from the High Priest to imprison Jews who became followers of Jesus. He was on the Road to Damascus with those letters to arrest Jewish Christians (Acts 9:1–2).
The first recorded Christian letter is found in acts 15:22–29 when the leaders of Jerusalem sent a letter to the Gentile believers to avoid offending the Jews. This letter was sent with Paul and other respected Christians.
The first Gospel by modern scholars is the Gospel of Mark with the author being the Apostle Peter or some other writer.
The first epistle written would be considered by the Apostle Paul to the Epistle to the Galatians. This letter is written by the Apostle Paul so it is not likely that it was a dictated letter.
SUMMARY: The first dictated Christian letter is the Jerusalem Letter to the Gentiles believers in Jesus Christ by the Christians letters in Jerusalem.
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