Steve Ray has a list of 35 loaded Questions for "Bible Christians" (quotation marks his)(link to the whole list). This is number 12/35. I'm trying to provide the answers in a common format, for easy reference.
12) How do we know, from the Bible alone, that the letters of St. Paul, who wrote to first century congregations and individuals, are meant to be read by us as Scripture 2000 years later?
2 Peter 3:16 As also in all his [Paul's] epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.
This is simply another way of phrasing the canon question that we've already addressed several times.