Class question from Jan 3 on the NT letters

Here is a very good excerpt extracted from an article included in the ESV Study Bible on "Reading the Epistles" (ESV Study Bible, pp. 2147-2149), that addresses a great class question from last week on why the NT includes so much of God's word in the form of letters:

The Epistles are not abstract philosophical or theological essays that explain the salvation accomplished by Jesus Christ. In almost every instance, they are addressed to specific situations facing churches. It is clear in reading Galatians, Colossians, 2 Peter, and Jude that the letters were written because false teaching had infiltrated the churches. Upon reading 1–2 Corinthians, we realize that Paul wrote in response to various problems in the Corinthian church. The letters are crafted to speak to readers as they face everyday life. In his first letter, Peter addresses readers who were suffering discrimination and persecution. Colossians responds to some kind of mystical teaching that promises readers fullness of life apart from, or going beyond, Christ. Philippians hints that the church suffered from some type of dissension and lack of unity. In the two Thessalonian letters, the church was confused about eschatology, and some believers were apparently becoming lax and failing to work hard. While many themes in Paul's thought are set forth in Romans, even that letter does not represent a comprehensive exposition of the gospel, for we do not find in the letter a developed Christological exposition (cf. Phil. 2:6–11; Col. 1:15–20), an explanation of Paul's eschatology (cf. 1–2 Thessalonians), or an unfolding of a Pauline doctrine of the church (see Ephesians; 1 Timothy; Titus). Ephesians may be a circular letter sent to a number of churches, in which Paul sets forth a more comprehensive understanding of the church, but even Ephesians lacks a complete exposition of all of Paul's theology. We must mine all of Paul's letters to determine his theology—and God, in his providence, has given us all the letters (and, of course, the whole of Scripture) so that we can understand the “whole counsel of God” (Acts 20:27).

In interpreting the Epistles, then, we should try to understand the specific circumstances that the original readers were facing. Upon reading Galatians, for instance, we see readily enough that Paul is responding to opponents who are subverting the gospel. Our understanding of Paul's purpose in writing Galatians is sharpened if we piece together the clues in the letter to reconstruct the views of Paul's opponents. We see that certain outsiders had infiltrated the church and were arguing that the Galatians must submit to circumcision and keep the OT law in order to be saved (cf. Gal. 1:7; 2:3–5; 3:1–14; 5:2–6, 12; 6:12–13). Paul contends vigorously that no one is saved by works of law but only through faith in Jesus Christ.

As readers of the Epistles today, we face a disadvantage that the first readers did not have, for they knew firsthand the situation that the letter writer addressed. Our knowledge of the circumstances is partial and incomplete. Reading the letters can be like listening to half of a telephone conversation: we hear only the writer's response to the situation in a particular church. Still, we trust that God in his goodness has given us all we need to know in order to interpret the Epistles adequately and to apply them faithfully.