Why is the study of letters important for our understanding of the Bible?
Because over 30 percent of the writing in the New Testament is contained in letters. Most are written by Paul Romans, 1 Corinthians, 2 Corinthians, Galatians, Ephesians, Philippians, Colossians, 1 Thessalonians, 2 Thessalonians, 1 Timothy, 2 Timothy, Titus, and Philemon. Hebrews was probably written by Apollos, and James, 1 Peter, 2 Peter, 1 John, 2 John, 3 John, and Jude are written by the name in the title.
Why do some call these epistles and not letters?
Deissmann insists (p224) that most of the letters in the New Testament are letters and not epistles.