An epistle is a long, formal letter. Several books of the Bible consist of epistles.
If you're used to emails and text messages, you might be surprised by the idea of the epistle — a letter that is very long and also very formal. An epistle isn't something you dash off in a few minutes while waiting for the bus. Epistles take time and thought to write, almost like doing a research paper. The Bible is full of many epistles — that's where many people have seen this word.