Q: How do we know there is no more scripture out there?
This question is interesting because it certainly raises a larger spectrum of questions than just about whether the canon--our list of Bible books--is closed. This question also raises some of the following:
Who had the authority to pick the books of the Bible?
Why were the books chosen that are in the canon?
Is the list open to further review?
The question also makes me think about all the documentaries I've seen on cable dealing with speculation arising from books like "The Gospel of Thomas" or "The Shepherd of Hermas". There are other letters or books from the biblical period that didn't make the list that were contenders in the debate over the canon.
SO....the way we know there is no more scripture....here's the best way I can answer it.
The canon--the list of what the Church agreed was scripture--was important for early Christians, as it became a capital offense to be caught with these books. The early church leaders needed to define what books were sacred and worth dying for.
Beyond that, a canon became increasingly important to define as various early church leaders, including some heretics, were devising lists of the books they considered authoritative. Many of these lists either left out books now in our canon or included some that somehow championed their sometimes obscure points of view. If you want to know more about one odd list, Google Marcion's canon.
As councils were convened--kind of like conventions of early church leaders held in different cities--Christians from all over the ancient Mediterranean world found that they were using the same books for worship and teaching. They agreed on a shortening list of books that they believed had the authenticating mark of Holy Spirit inspiration and, thus, authority.
Finally, the list of books we now use was agreed upon in 393 AD, though no book is thought to have been written after the first century.
We don't imagine there are more books out there, as the early Christians debated and considered a pretty big list, but only considered those books that they had used as a part of worship and teaching.
I hope this answer helps in coming to a fuller understanding of how our canon came to be. Thanks for the question!
Check out more posts from the 31 Questions series on subjects as varied as marijuana, infidelity, 666, the eternal destiny of children who die, dealing with discouragement, cremation, co-habitation, baptism, suicide, prayer, and even speaking in tongues!