Types of Early Christian Letters
When I began studying the early church I was astounded by the amount of documents that had been written by the first few generations of Christians. Eusebius refers to many of these documents in his Church History. They include copies of theological treatises and sermons, commentaries on books of the Bible by Origen, as well as a few histories, but many of the documents Eusebius mentions are letters written by early Christians.
Some letters were defences written to emperors and governors in which eloquent Christians argued that Christianity was not a threat to Roman society. But most of the letters were written from one church to another, or from one bishop (or group of bishops) to another, or from people writing to a church leader or teacher for advice. Eusebius even quotes from a letter purporting to have been written by King Abgar V of Edessa to Jesus . . . along with a reply! (1.13.1-9) There were also letters written in prisons by Christians about to be martyred (5.3.4). Furthermore, many of the treatises and sermons written by early church theologians and bishops were written in the form of a letter. These letters expressed teachings, encouragements, and concerns about church life and Christian beliefs.
Correspondence between churches and between Christians was vital to the early church, Deacons, and church agents, carried letters throughout the Roman empire, maintaining networks of communication between the churches (e.g. Tychicus in Eph. 6:21; Col. 4:7; 2 Tim. 4:12; Tit. 3:12). [More about deacons and letter carriers, including Phoebe, here.]
The Survival of Early Christian Letters
In book 5 of his Church History, Eusebius mentions the letters and other works of Irenaeus, and he goes on to say that, “Many works by churchmen of that time [i.e. the late 100s] are still widely preserved, and I have read them myself” (5.27). Eusebius then lists some works by lesser known names and adds that “there are many other orthodox writings” (5.27, my italics).
Eusebius, who was bishop of Caesarea Maritima in the early 300s, sourced the documents he used, including letters, from a library in Aelia (i.e. Jerusalem). He mentions this library in book 6 of his Church History where he writes:
During this period [early 200s] flourished many learned church men whose correspondence still survives and is easily accessible in the library at Aelia established by Alexander [a bishop of Jerusalem who was martyred in 256 ]. It was here that I was able to gather the material for this book (6.20).
It seems that there was a lot of writing going on in the early church, especially the writing of letters. S.K. Stowers notes, “Something about the nature of early Christianity made it a movement of letter writers. We possess more than nine thousand letters written by Christians in antiquity.” Nine thousand letters (many of which are mundane) have survived to the present day, but many more thousands of letters have been lost over time.
Many of the letters that Eusebius used are lost except for his quotations; copies of some other letters, however, are extant. The surviving letters that Christians today are most familiar with are those that were collated and included in the New Testament. About half of our New Testament consists of letters written to churches and individuals.
Customs of Writing and Reading Letters
Letter writing in the first century was an expensive and time consuming process. When a person wrote a letter, he or she often used an expert amanuensis (secretary), as letter writing was a specialised skill involving standardised rhetorical techniques. A letter often went through several drafts before a carefully crafted letter was finalised. Some amanuensi are mentioned in the New Testament, for example: Tertius (Rom. 16:22) and Silas (1 Pet. 5:12). Some of the people Paul mentions in his greetings may also have helped Paul to write and craft his letters, for example: Sosthenes (1 Cor. 1:1) and Timothy (Phil. 1:1).
Paul, and other letters writers, expected that their correspondence would be read aloud in church meetings (cf. 3.3.6). They also expected that more copies would be made and circulated among other churches. For instance, many scholars believe that the New Testament letter to the Ephesians was intended for other churches of Asia Minor, and not just Ephesus. Likewise, towards the end of the letter to the Colossians is the instruction: “After this letter has been read to you [Colossians], see that it is also read in the church of the Laodiceans, and that you in turn read the [now lost] letter from Laodicea” (Col. 4:16).
Eusebius writes about the custom of reading letters aloud in church meetings. In book 3, he mentions that First Clement (a letter written by the church at Rome to the church at Corinth in around 96), “was read publicly in many churches both in days of old and in our own” (3:16). In book 4, Eusebius quotes from a letter by Dionysius (the bishop of Corinth until around 172) written to Soter (bishop of Rome from 167 t0 174). In this quotation Dionysius mentions both a letter from Soter and First Clement, and writes, “We read your letter today, the Lord’s Day, and shall continue to read it frequently for our admonition, as we do with the earlier letter Clement wrote on your [i.e. the church at Rome’s] behalf” (4.23.11).
In the first two centuries of the church, most congregations met in homes. Some congregational leaders had little training in Christian doctrine, so copies of letters from apostles and bishops played an important part in the worship and education of congregations. Letters were read aloud to provide inspiration, encouragement, teaching, and correction.
I continue to be amazed by the amount of letters, and other documents, that were composed by the early Christians. I am jealous of Eusebius who had access to the resources of the library at Jerusalem, but I imagine that he would be even more jealous of the many surviving documents, including the all-important books and letters of the New Testament, that are now freely available on the internet—books and letters that continue to inspire, encourage, teach, and correct.
 The Corinthians had written to the apostle Paul asking for advice. In First Corinthians, Paul occasionally quotes from the letter he received from them. Some of these quotations are: “‘it is not good for a man to touch a woman” in (1 Cor. 7:1); “we all possess knowledge” in (1 Cor. 8:1); “there is no resurrection” and “Christ has not been raised” in (1 Cor. 15:12,14). Some scholars believe that 1 Corinthians 14:34-35 may also be a quotation.
 Maier writes that,
The story of Abgar’s correspondence with Jesus, however, sensational, must be regarded as apocryphal. There is no doubt that these documents were in the archives at Edessa . . . and that Eusebius himself saw and translated them himself. . . . the spurious nature of these documents is indicated by Jesus referring to items written about him at a time when they could not yet have been written down. Eusebius was not a critical historian in the modern sense.”
Paul L. Maiers, Eusebius–The Church History: A New Translation with Commentary (Kregel Publications, 1999, 2007), 53-54.
 S.K. Stowers, Letter Writing in Greco-Roman Antiquity (Philadelphia: Fortress, 1986), 15.
 Many of these letters are brief and mundane in their content, yet they still provide glimpses into the lives of early Christians. (See comment below for more on this.)
 Some documents (e.g. the Didache) were lost, but were rediscovered in the nineteenth and twentieth centuries by archaeologists digging in Egyptian sands, and by people scouring through ancient collections of scrolls and books in monasteries and libraries, etc.
 Ben Witherington, 1 and 2 Thessalonians: A Socio-Rhetorical Commentary (Grand Rapids: Eerdmans, 2006), 260.
 Eusebius states that girls skilled in penmanship were among those who served as copyists of Origen’s Bible commentaries (6:23.2). Perhaps educated girls also copied other material, such as letters, for the church. While I’m on the subject of Origen, here is another piece of information from Eusebius: Origen taught “religious matters”, or “the study of divine things”, to women as well as men (6.8.1).
 In his commentary on Ephesians, Clinton E. Arnold writes that this letter “was intended in the first instance to circulate in nearby villages, and possibly to churches in cities as far away as Smyrna, Miletus, and the Maeander and Lycus valleys.” Ephesians (Grand Rapids: Zondervan, 2010), 29.