Thank you for the interpretative question on Paul’s writing, “With all the myriad of letters the apostle Paul wrote, why were the thirteen epistles of his (were) the only ones that found their (way) into the Bible?”
I may be reading too much into the question. I am uncertain whether the question is asking, “Why are only thirteen epistles of Paul are in the New Testament?” or “Why are there only thirteen epistles of Paul in the New Testament, that is excluding the General Epistles as I-II Peter, James, Hebrews, Jude and Revelaton?” I assume the former question of why aren’t there more letters of Paul in the New Testament. as I believe you know that there are the General Epistles.
One can’t state specifically why the other Pauline writings were not included in the New Testament just as there were other Old Testament prophets whereby their messages were not in the Old Testament. If one was to conjecture, here are some reasons why the other letters were not preserved to be in the New Testament:
1. The Gentile churches preserved the more relevant or applicable writings of the Apostle Paul than some of his other writings. The value is in the content of the Apostle Paul’s writing.
Not all writings of the Apostle Paul are of the same equal value. Just as we received mail or text messages, there are some text messages that we keep in our phone while others we delete due to lack of importance or relevance to us.
The early Christians needed instructions as to how to organize and how to deal with certain situations. Since Paul was considered the apostle to the Gentiles, the instructions given by him helped the early churches in dealing with similar situations in that church that Paul wrote to.
For example, the “First” Letter to the Corinthians provides clear instruction to the Church of Corinth. Other churches found that they had the similar problems. Thus, this particular letter was more highly treasured for its helpfulness to the Church. The letter that preceded “First” Corinthian was not preserved or “lost” through time. See I Cor. 5:9–11 of a citation of a previous letter and the letter that that was read to them.
2. The Gentile churches may have made more copies of some of the Apostle Paul’s writing than others.
It would seem that the early Christians heard about a letter from the Apostle Paul and how helpful that was to that church that copies of the original letter were made or ask that his letter be read in their churches.
Thus, more copies of certain letters made than other letters. In Colossians 4:16, the Apostle Paul asks that this letter be read in Laodicea. In return, the letter of Laodicea was to be read to them. The desire of the early churches to have the letter or a copy of that letter was in demand by the churches.
3. The representatives of the Apostle Paul as Timothy and Titus affirmed or added their wisdom in addressing the needs of the Churches. They had learned from the Apostle Paul and Paul was trusting them to resolve the early church problems. The letter was an instruction to the early churches and an affirmation that his representative words were in accordance with Paul’s instruction.
It may be that over time that some Christians used the name of the Apostle to give apostolic authority to their “letter” or claim that they had as much authority as Paul. See II Corinthians 11 that some Christians even claimed that they were “superapostle!”
4. The early churches had to discern and decide which letters of Paul’s were authentic, while others were forgery.
It appears that some letters of the Apostle Paul were readily accepted as being from him, while others letters were suspect or considered forgery. It is likely that certain letters of Paul were already widely received as being from him while others may have been contested. The uncontested letters, if I may use that term, were viewed with the wisdom of God and highly treasured. They became part of the early churches reading and instruction, while others were less read or rejected as forgery.
5. The early churches may have found that some letters were of a personal nature by the Apostle Paul to a particular person, not addressing the need of a church.
The letters to Timothy and Titus were of a personal nature and yet very applicable to the churches. There may have been other writings to particular church leaders that may not be deemed as applicable to the early church or to a person’s particular situation.
6. The early churches were persecuted by the Romans which at times sought to destroy the writings of the Christians.
If my memory serves me correctly, Emperor Diocletian sought to revive the worship of the Greek god and goddess. The writings of the Christians were burned, depending on the area.
It may also be that through time the churches developed their own handbook of Christian conduct as the Didache.
Again, I do not know the particular reason why some letters of the Apostle Paul are considered part of Scripture while others were not. The above answers are conjectures.
Summary: God in his sovereignty allowed certain writings of the Old and New Testament to be preserved and gathered together to form the canon.
-Kingston