The Bedford Glossary of Critical and Literary Terms - Ross Murfin 2018
Epistle: A letter. As a literary form, an epistle is generally restricted to a formal (not conversational), thoughtfully composed letter that is intended for a distant individual or group of people. Although some of the most famous epistles come from the New Testament of the Bible, any such formal letter, regardless of religious or moral intent, is an epistle. Nearly all epistles are written in prose, but a few have been composed in verse, including a number by English poet Alexander Pope.