Epistle – A command or commission, whether verbal or in writing, sent as a message or dispatch.
God used this word 16 times in Scripture: Never to Israel in their Masoretic Text Old Testament or Koine Greek Old Testament; Twice to the Body of Christ in the transition book of Acts; 12 times to the Body of Christ in the Gospel of Grace (Romans thru Philemon); and twice to Israel’s Little Flock in their tribulation book of II Peter.
Being an epistle does not make a writing Scripture. An epistle is a command or commission, as opposed to a letter or tidings.
The first epistle recorded in Scripture was in the form of a letter sent by Jesus’ apostles and the elders of the Little Flock in Jerusalem to Antioch Syria. It was carried by Paul and Barnabas, and they were accompanied by Judas surnamed Barsabas, and Silas.
The second epistle recorded in Scripture was in the form of a letter sent by Claudius Lysias to governor Felix in Rome, carried by a Roman Centurion. The epistle was delivered with Paul in custody.
In the late 1200s the Catholic Church mystified this word by changing it to mean “letter from an apostle forming part of canonical scripture”. In the mid-1500s they called their corrupt additions to their corrupt bible a “book containing epistles read in the Mass”.