According to Webster's Dictionary...

It has come to my attention that most people don't know what an epistle is. Rather than flex my lexical (?) muscles, I'll let my good friend Noah Webster handle this one.

e·pis·tle Audio pronunciation of "epistle" ( P ) Pronunciation Key (-psl)
n.

1. A letter, especially a formal one. See Synonyms at letter.
2. A literary composition in the form of a letter.
3. Epistle Bible.
1. One of the letters included as a book in the New Testament.
2. An excerpt from one of these letters, read as part of a religious service.


That asshole Webster is charging to use his site, which isn't even as good as dictionary.com. So that's where this is from. Be that as it may, I hope this has cleared up any confusion.