The word “epistle” comes from the Greek word epistolē, which means “letter.” We typically associate that word with personal correspondence, but a “letter” might refer to all sorts of documents (commercial, legal, governmental, etc.).
The letters we see in the New Testament are both personal and formal. On the one hand, the writers have a personal attachment to the recipients. Paul, for instance, wrote letters to churches he had founded. Peter and James wrote more general letters to groups of believers who were under persecution. Given the fact that some New Testament letters were aimed at audiences scattered about and not in one location (James 1:1) or were shared with other churches (Col. 4:16), New Testament letters also had a formal feel to them. Their content was considered important for believers who had not been the initial addressees.
In our age of instant and scattershot communication, the art of letter writing has largely been lost. A good letter has discernible components and structure. I can remember learning how to write a letter in grade school. My teacher taught us that letters began with a salutation (Dear . . .). Instead of getting right to the point, our salutation was supposed to be followed by some sort of light banter to hopefully set the right tone. We might want to include a preview to what the letter was ultimately about. Then came the body of the letter—whose coherence was fostered or undermined by clear or confused paragraphing. When we were bringing things to a close, we were taught to leave the reader a final reminder so that the main purpose of the letter wouldn’t get lost. Then we closed with “Sincerely” if formal or “Love” if appropriate and signed our names.
New Testament letters also followed patterns. Most of Paul’s letters open with a personal salutation, greeting, and blessing or note of thanksgiving. Here is an example:
Paul, called by the will of God to be an apostle of Christ Jesus. . . . To the church of God that is in Corinth, to those sanctified in Christ Jesus. . . . Grace to you and peace from God our Father and the Lord Jesus Christ. I give thanks to my God always for you because of the grace of God that was given you in Christ Jesus. (1 Cor. 1:1–4)
Paul would then launch into the body, which he divided into a teaching section and an exhortation/application section (cf. Eph. 1–3, 4–6). He often closed by sending more personal greetings to friends by name (Rom. 16:3–16).
Knowing what to expect is helpful for those times when the writer breaks the expected conventions. Recognizing the norm helps draw our attention to the exception, which is usually what the writer intended to do.