The majority of the letters of the New Testament were written by Paul, which are called the Pauline Epistles.
The word "epistles" literally just means letters, and the name Paul is used as an adjective to describe the letters: Pauline Epistles = Paul's Letters.
Consequentially, Paul is viewed as a major source of biblical theology, because he is a major author of the New Testament and teaches about numerous subjects that still pertain to modern Christians. However, it is important to realize that there are subjects included in Paul's letters that do not pertain to modern Christians, because he was teaching about subjects that were cultural-based to "specific people," in a "specific time-period," who existed in a "particular culture."
A quick cultural example in Paul's letters is found in Romans 16:16, where Paul says, "Greet one another with a holy kiss." In many modern cultures, where kissing is reserved for family or is viewed as something used only for romantic situations, such a suggestion might be sinful. To the 1st century Roman church, Paul says, "holy kiss," but if Paul were to write to a modern church, he would likely say, "Greet one another with a friendly handshake."
Another great cultural example in Paul's letters is when Paul talks about subjects like Head Coverings in 1 Corinthians 11, which makes little sense to modern Christians who live in a culture where women never cover their heads for any reason other than maybe it's cold. Many people try to use this as a metaphor, but Paul is not speaking metaphorically. He is speaking literally to a group of people who lived in a culture where this was an issue where believers were uncertain of how to behave.
Paul's letters have plenty of other cultural examples in them, which are generally easily identifiable.
In reality, if Paul were to write a letter to a modern culture, there are actually situations where he might literally tell modern believers to behave in the exact opposite manner that he told 1st century believers. And there's nothing wrong with that fact; it's just because the culture is different, but it is important to identify what is cultural and what transcends culture.
Paul's letters were organized by early Christians according to their length (based on words, not chapters), with Romans being the longest of his letters, followed by 1 & 2 Corinthians. Titus and Philemon are last because they are his shortest letters.
The following is a list of the Pauline Epistles, ordered canonically (in the order they are listed in the Bible). Each letter is summarized by chapter, in order to break down Paul's thoughts into manageable pieces.
Note: The author of the book of Hebrews is unknown. Some people believe that
Paul wrote this letter, but this perception has long since been viewed as unsubstantial. There simply is not enough evidence to determine who the author is.