This is really like asking if we might discover a previously unknown letter of the alphabet.
The Bible is not an open-ended collection of books. The content of the Bible, that is, the ‘canon,’ is a fixed list of books that was defined in the early centuries by the Jews (in the case of the Old Testament) and the Church (in the case of the New Testament). It would be possible to find additional parts of the Bible if the ancient lists contained titles for which the books are missing. However, that is not the case.
The Old Testament
There were two versions of the Old Testament in use in Judaism before the Church came into existence.
The Hebrew version, which was used in Palestine, contained all the books that you find in the Old Testament of a Protestant Bible. The list of books in the Hebrew Bible is called the Palestinian Canon.
Centuries before the Christian era began, the Jewish community in Alexandria (Egypt) translated the Hebrew Bible into Greek, but their collection had more books in it. Their list of books is called the Alexandrian Canon. Since about 70 scholars were involved in the project of translating the Hebrew Bible into Greek, their translation is called the Septuagint, from a word meaning ‘seventy.’ The books that are on the Alexandrian list but not on the Palestinian list are called the Apocrypha.
Jewish synagogues outside of Palestine and all ancient Christian churches used the Septuagint or translations of the Septuagint as their Old Testament. When the New Testament quotes the Old Testament, it usually quotes from the Septuagint instead of retranslating the quotation from the Hebrew Bible. All modern Christian Bibles give the Old Testament books their Septuagint names and place them in the order they appear in the Septuagint.
Some modern Protestants reject the Apocrypha. Roman Catholics accept the Apocrypha as having the same authority as the other Old Testament books. All other Christians, including many Protestants, use the Apocrypha as a liturgical resource and for instruction in faith and morals, but not as a source of doctrine.
None of the books of the Hebrew or Palestinian canons of the Old Testament are missing. Hence, it is not possible to find any additional Old Testament books.
The New Testament
By AD 130, Christian writers had quoted from most of the books in our New Testament, using them as Scripture. Some of the very small epistles and Revelation were in dispute in some areas. There was no serious need for the Church to issue an official list of New Testament books until Marcion, a heretic, challenged huge chunks of the New Testament in AD 140. The Church asserted the validity of the New Testament books over the years. Finally, in the east in AD 367, Bishop Athanasius issued a list of New Testament books in his Easter Letter. In the west in AD 382, a local council in Rome issued the same list. Since there were no disputes about which books are in the New Testament, the matter never came up at an Ecumenical Council.
None of the books in the New Testament canon are missing. Therefore, it is not possible to find any additional New Testament books.
Other Ancient Religious Literature
As we rediscover ancient manuscripts, we regain lost religious works that the ancient Church knew about. Some were accepted by the Church as instructive but not scriptural, much as we might use the writings of C.S. Lewis today. These books include the Didache, the Epistles of Clement, or the Shepherd of Hermas. Others were considered by the early Church to be spurious or heretical. For example, Bishop Eusebius Pamphilus of Caesarea wrote a History of the Church that was published in AD 325, in which he reported that the Church regarded the Gospel of Thomas as a heretical work. However, the actual text of the Gospel of Thomas was lost and only recently discovered. We can now verify that the Church was entirely consistent when it failed to get enthusiastic about the Gospel of Thomas. Some people have made a big hullabaloo about the Gospel of Thomas, which mystifies me. Why don’t they get equally excited about, say, the Acts of Paul and Thecla, which was much more popular in the early Church, but not accepted as scripture? The only reason I can think for this uneven treatment is that the Acts of Paul and Thecla was with us all along and the Gospel of Thomas has the mystical aura of having been lost for centuries and only recently being rediscovered. Of course, another reason could be that the Acts of Paul and Thecla teach sexual abstinence, even in marriage, while the Gospel of Thomas is conducive to the modern New Age mindset. What we are discovering are new insights into the theology of the early Church.
Here is a quick summary of important terms:
- In general, a by-law of the ancient Church. In this context, a list of books that are recognized as being Holy Scripture.
- Palestinian Canon
- The list of books in the Hebrew Bible, identical with the Jewish Bible and the Protestant Old Testament.
- Alexandrian Canon
- The list of books in the ancient Jewish translation of the Hebrew Bible into Greek, identical with the Septuagint and the Old Testament of the Roman Catholic Church and the Eastern Orthodox Churches.
- The name of the ancient Jewish translation of the Hebrew Bible into Greek. It was translated in Alexandria, Egypt, by about 70 Jewish scholars with support from the Temple in Jerusalem. It is called the Septuagint from a word that means ‘seventy.’ In the first century, it was the Bible of Jews who lived outside of Palestine and it was the Bible of the Christian Church. It is still the official Old Testament of some Orthodox Churches today.
- The collective name for the books that are in the Septuagint but are not in the Hebrew Bible. This name is mainly used by Protestants who do not regard these books as fully canonical or not canonical at all. (The terms Apocrypha and deuterocanonical books are basically synonyms. They refer to the same books, but they express a different opinion about them.)
- Deuterocanonical Books
- The collective name for the books that are in the Septuagint but are not in the Hebrew Bible. This name is used by Roman Catholics, some Anglicans, and others who accept these books as canonical. The word deuterocanonical means ‘second canon,’ meaning that these books came to the canon after the other Old Testament books. (The terms deuterocanonical books and Apocrypha are basically synonyms. They refer to the same books, but they express a different opinion about them.)
- New Testament Apocrypha
- The collective name for ancient Christian religious literature that is not in the New Testament.
- The collective name for ancient Jewish religious literature that is not in the Jewish Bible or other recognized rabbinical works.