The main reasons surround attribution and standards. The requirements for a book to be in the bible was that it was written by a Prophet, Apostle or one under the authority of an Apostle as in Mark and Luke. The writing had to be written in Koine Greek and Hebrew and was found to be inspired by God, which is to say it did not conflict with any of the other writings in the spirit of truth and was found to support all the scriptures with precepts.
The book of Enoch, Thomas and Barnabs are considered pseudepigraphy, falsely-attributed works, texts whose claimed author is not the true author, or a work whose real author attributed it to a figure of the past.
Pseudepigraphy covers the false ascription of the names of authors to particular works, even to authentic works that make no such claim within their text.
There is something referred to as the seven levels of authenticity of scripture ranging from literal authorship, meaning written in the author's own hand, to outright forgeries, these are:
- Literal authorship. A church leader writes a letter in his own hand.
- Dictation. A church leader dictates a letter almost word for word to an amanuensis, which is a person employed to write or type what another dictates or to copy what has been written by another, it also refers to a person who signs a document on behalf of another under the latter's authority, like Mark and Luke under Peter and Paul's authority.
- Delegated authorship. A church leader describes the basic content of an intended letter to a disciple or to an amanuensis.
- Posthumous authorship. A church leader dies, and his disciples finish a letter that he had intended to write, sending it posthumously in his name.
- Apprentice authorship. A church leader dies, and disciples who had been authorized to speak for him while he was alive continue to do so by writing letters in his name years or decades after his death.
- Honorable pseudepigraphy. A church leader dies, and admirers seek to honor him by writing letters in his name as a tribute to his influence and in a sincere belief that they are responsible bearers of his tradition.
- Forgery. A church leader obtains sufficient prominence that, either before or after his death, people seek to exploit his legacy by forging letters in his name, presenting him as a supporter of their own ideas.
The new testament list of pseudepigrapha is as follows
- 3 Maccabees
- 4 Maccabees
- Assumption of Moses
- Ethiopic Book of Enoch (1 Enoch)
- Slavonic Second Book of Enoch
- Book of Jubilees
- Gospel of Barnabas - not the Epistle of Barnabas or the Acts of Barnabas.
- Gospel of Thomas
- 3 Baruch
- Letter of Aristeas
- Life of Adam and Eve
- Ascension of Isaiah
- Psalms of Solomon
- Sibylline Oracles
- 2 Baruch
- Testaments of the Twelve Patriarchs