I’m afraid I cannot answer your question, but I can advise you to look it up. There’s bound to be explanation; there are so many other books that have banned from the Bible whether it’s because of the content or they gave religious leaders the “red flag” or simply because they didn’t fit. I wouldn’t be surprised if one or two were fiction.
There are some books that I would have made canonical if I were in charge, but fortunately I am not because I think that the Church has done quite well without me for the last 2000 years. But as far as it not being considered I can think of a couple of possible reasons why. First is that it was not circulated anywhere near as much as the other books were, meaning that it was not received universally as Scripture to make it considered. There were other writings that did not make it into the canon that did get circulated a lot and even received as Scripture in some manuscripts, writings such as Clement, Barnabas, and Hermas. Another reason that may have played a part is that it quotes the book of Matthew a couple of times which may have made it to be considered a post Apostolic writing. If it had been placed in the canon it would have made the job of a Catholic apologist a little easier when defending correct doctrine, but it is still valuable because it does show us that the thought of the early Church was purely Catholic.
The fundamental criterion of the books of the New Testament is apostolic origin. The didache, which is a testimonial to early church teachings, was not deemed to be itself of apostolic origin. In other words, it was not written or inspired or authorized by an apostle because it was not “received” as such. The Church did not “authorize” the books of the Bible. It is a witness to their authenticity.
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