Epistole is a letter or written communication — it gives us the English word 'epistle.' In the NT, it refers to the authoritative letters written by apostles to churches and individuals. Twenty-one of the NT's 27 books are epistles. The word was common in the Greco-Roman world for official and personal correspondence.
Paul elevates the letter as a theological medium. In 2 Corinthians 3:2-3, he calls the Corinthian believers themselves a letter 'written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.' The congregation becomes a living epistle that the world reads. This democratizes the authoritative word: Scripture was delivered in letters; the church itself is now a letter. The authority of the apostolic epistles gives the church its doctrinal foundation (2 Peter 3:15-16 affirms Paul's letters as Scripture).