Since then there have even been manuscripts written as close as 100 years ago, attempting to add to the revelation of the Bible. Some also claim to be based upon genuine documents of Christian antiquity.
The 'Dead Sea Scrolls' were discovered between 1947 and 1956 in the remains of an ancient community called Khirbet Qumran located on the Northwest shore of the Dead Sea, in the middle east. Some of the writings were of books that we recognized like The prophet Isaiah, Psalms etc., others were of books that we were not aware existed; the book of Adam and Eve (detailing their life after expulsion from the garden of Eden), The book of Enoch (supposedly important because Enoch did not see death but was translated), the gospel according to Thomas (the disciple) and Mary (chronicling the life of Mary the mother of Jesus) just to name a few.
Upon reading most of these books it becomes obvious very quickly why they have not been adopted into the collection of what we call 'sacred scripture.' But how should we know if these books are necessary or if some new set of books is found tomorrow weather they are trustworthy enough to live your life by?
Well, first of all we have Moses, the lawgiver. The man whom we can say was moved by the Holy Spirit to write Genesis-Deuteronomy. It is this Moses whom the Lord Jesus was to be likened after. (Deuteronomy 18:15-19)
When Jesus came in the flesh He told the people that, He did not come to destroy the law but to Fulfill it! (Matthew 5:17-18)
It is not therefore an exaggeration to say that when Moses ended His writings the WHOLE story had been told. The 30th chapter of Deuteronomy speaks about the end times and what will happen when Israel is established as a nation (which has not yet happened contrary to popular belief).
The rest of the Bible from Joshua to Malachi, only builds on the foundation that God laid through Moses. The different prophets analyze and zoom in on the timeline laid by Moses, as well as expounding on with greater detail the things which must happen. The New Testament from Matthew to Revelation is the Messiah, and his Apostles testifying that the things which Moses (and subsequently the prophets) wrote was/is the Truth. I would say that in the gospels 95% of the things Jesus said and did, it is written, was done so that the word of the prophet might be fulfilled. EVERYTHING, from the place Jesus was born to the very last words that he uttered on the cross was written by one or several of the prophets.
Think about this: The Apostle Peter, the man whom the Lord left the keys of the kingdom with, the man who with 11 others walked with the Lord, spoke and ate with Him, asked questions of Him. For 3.5 years Peter saw with his own two eyes Jesus walk on water, feed thousands with a few loaves of bread and a few fish MORE THAN ONCE! Not to mention the countless miracles, His sermon on the mount, and His condemnation of the religious leaders of the day. After all that we see Peter confess that even though he was an eyewitness to all these things, we all have, a MORE SURE word of Prophecy! If the words of the prophets -who lived anywhere from 200 to 2000yrs before Jesus came in the flesh- are More Sure than the words of eyewitnesses, that is as sure as it gets!
Jesus said the words that He spoke were spirits (John 6:63). Coming from Jesus' mouth would make them holy spirits. If words are spirits then we should do as John instructed us to try every spirit, if it be come from God (1John 4:1). The same way we know if a man is of God based on if what he says can be found in the pages of scripture, we know that if a piece of Literature is to be considered inspired by the holy ghost it should also agree with universally recognized scripture.
LET IT BE KNOWN: If a new (or old) manuscript was to surface, claiming authenticity among the Christian scriptural catalog all we have to do is sit it side by side, next to what is already written and see if it agrees with what we know for sure is Holy.