"[Paul] writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction" (2 Peter 3:16).
Which books from the ancient Jewish culture are Holy Scripture? For us that seems a very difficult question, but at the time the books were written it was not. The Holy Spirit confirmed the writings of Moses, the prophets, David, and the apostles, with signs and wonders. The Spirit caused believers to recognize instantly the voice of the Master in these writings, and they were immediately added to the list of sacred books. Peter speaks of Paul’s letters as part of Holy Scripture in 2 Peter 3:16. The list of inspired books of the Bible is called the “canon” of Scripture.
Satan tried to confuse the issue in the second century A.D. First, he worked through the gnostic heretics to produce a lot of obviously fake gospels and letters. The church never gave these any serious consideration.
Second, Satan worked through the heretic Marcion to produce another canon (list) of Scripture. Marcion hated the “wrathful God of the Old Testament” and made up a list that contained only the writings he thought were nice and sweet. In response to this, the church took an official look at the Scriptures she had received and produced her own list, which is the list we still have today. The church was not making some kind of scientific determination so much as she was saying that these have always been the books, and there is no doubt about them.
But Satan tried one more ploy. He raised questions about Hebrews, Jude, and 2 Peter, and he tried to get the church to accept 1 Clement and the Shepherd of Hermas. In this way, he could muddy the waters of the Canon. But both Clement and the Shepherd make a clear distinction between themselves and Scripture, claiming only to be uninspired sermonic literature, so the church was not tempted to add them to the Canon. Also, in the course of discussion and consideration, it became clear that Hebrews, Jude, and 2 Peter stood apart from uninspired writings and resembled Scripture, never contradicted the other Scriptures, and had always been received as Scripture. So after a moment of wavering, the church returned to what she had always known to be the case, and kept those three books in the Canon.
Jesus sent the Holy Spirit to guide the church into all truth. Read 2 Timothy 3:17 and know that we have a completed Bible that is a seamless garment from beginning to end.