who preserved Paul's letters before they became Scripture?
Paul's letters were not lost because someone providentially recognized their importance and protected them. They were kept intact for over three hundred years, until the Church Fathers gathered them together and placed them into what is called Scripture. This preservation was itself miraculous.
Les explains that Paul's various letters went out to congregations in the ancient world where there were no copy machines and no instant communication, yet they did not get lost. Someone was providentially aware that these letters were important and protected them. They were kept intact for three hundred and some years before the Church Fathers put them together into what we call the Scripture. Les points out this is miraculous because these were just simple letters sent to a congregation, with only one copy, and no laborious handwriting of multiple copies. The letter especially was to be circled among all the churches in the area around Ephesus, and yet it never got lost. Les calls this a supernatural book.