HERMENEUTICS: What is the one thing that all of the New Testament epistles have in common?

I'm reading for the second time How to Read the Bible for All Its Worth by Gordon D. Fee & Douglas Stewart. Here's an excellent excerpt concerning the nature of the New Testament epistles:

There is one thing that all of the epistles have in common, and this is the crucial thing to note in reading and interpreting them: They are all what are technically called occasional documents (i.e., arising out of and intended for a specific occasion), and they are all from the first century. Although inspired by the Holy Spirit and thus belonging to all time, they were first written out of the context of the author to the context of the original recipients. It is precisely these factors— that they are occasional and that they belong to the first century— that make their interpretation difficult at times.

Above all else, their occasional nature must be taken seriously. This means that they were occasioned, or called forth, by some special circumstance, either from the reader’s side or the author’s. Almost all of the New Testament letters were occasioned from the reader’s side (Philemon and perhaps James and Romans are exceptions). Usually the occasion was some kind of behavior that needed correcting, or a doctrinal error that needed setting right, or a misunderstanding that needed further light.

Most of our problems in interpreting epistles are due to this fact of their being occasional. We have the answers, but we
do not always know what the questions or problems were— or even if there was a problem. It is much like listening to one end of a telephone conversation and trying to figure out who is on the other end and what that unseen party is saying. Yet in many cases it is especially important for us to try to hear “the other end” so that we know what our passage is a response to.

 One further point here. The occasional nature of the epistles also means that they are not first of all theological treatises, nor are they summaries of Paul’s or Peter’s theology. There is theology implied, but it is always “task theology”— theology being written for or brought to bear on the task at hand. This is true even of Romans, which is a fuller and more systematic statement of Paul’s theology than one finds elsewhere. But it is only some of his theology; in this case it is theology born out of his own special task as apostle to the Gentiles. It is his special struggle for Jew and Gentile to become one people of God, based on grace alone and apart from the law, that causes the discussion to take the special form it does in Romans and that causes “justification” to be used there as the primary metaphor for salvation. After all, the word “justify,” which predominates in Romans (fifteen times) and Galatians (eight times), occurs only two other times in all of Paul’s other letters (1 Cor 6: 11; Titus 3: 7). Thus one will go to the Epistles again and again for Christian theology; they are loaded with it. But one must always keep in mind that they were not primarily written to expound Christian theology. It is always theology applied to or directed toward a particular need.

Fee, Gordon D.; Stuart, Douglas (2009-10-14). How to Read the Bible for All Its Worth (Kindle Locations 980-1003). Zondervan. Kindle Edition.