"We have four independent documents, three of which cover much of the same material (Synoptic Gospels).
There are no known competitors for authorship of the Synoptic Gospels (Matthew, Mark and Luke.)
Matthew and John were disciples and therefore eyewitnesses.
Christian writer Papias affirmed in AD 125 that Mark had carefully and correctly recorded Peter's eyewitness observations. (Mark was the earliest gospel recorded.)
The standard scholarly dating is that Mark was written in the 70's (a mere 40 years after the Resurrection), Matthew and Luke in the 80's and John in the 90's - all within the lifetime of eyewitnesses! Many believe the dates are even earlier, within 30 years of Jesus' death. The above dates are conservative. While these dates seem late, they are not. The two earliest biographies of Alexander the Great more than 400 years after his death, yet historians consider them to generally trustworthy. In other words, the first 500 years kept Alexander's story pretty much intact; legendary material developed over the next 500 years. The Creed Paul is given dates back to within 2-4 years of the events. (Crucifixion around AD 30, Paul's conversion AD 32.)
Textual Criticism is defined as: the science of comparing and classifying the manuscript evidence for an ancient document. Scholars of almost every theological persuasion attest to the profound care in which the New Testament books were copied. In the original Greek alone, there are over 5,000 manuscripts and manuscript fragments of portions of the New Testament, preserved from the early centuries of Christianity.
There is an unprecedented number of copies of the New Testament that have survived. Scholars wish they had 1/10 the number of documents for other ancient literature! There are only 9 copies of Josephus' The Jewish War, and these copies were written in the 10th - 12th centuries. Yet, this work is considered to be historically accurate.
Astonishingly, the second runner-up for the largest number of ancient documents goes to Homer's Illiad, which was the sacred literature of the ancient Greeks. There are fewer than 650 of these manuscripts and many are quite fragmentary.
Fragments of John's gospel have been found that date as early as 150 AD. This find has literally rewritten popular views of history.
Two of the almost complete New Testament documents date back to the 4th Century! These two texts play an important role in the NIV Bible. This means we have documents within 2 generations of the events, unlike 8 or 10 centuries for much of our other historical documents. In addition to the over 5,000 Greek manuscripts, there are about 24,000 other ancient New Testament documents in other languages found in areas such as Egypt and Ethiopia.
The evidence is simply overwhelming. The huge number of documents, many dating within 2 generations of the events, have allowed us to study them side-by-by side, comparing and analyzing the New Testament. While many documents are fragments, there is no part of the NT not represented by multiple documents. The history of the world is based on far fewer manuscripts and evidence!
For further study, consider the following books:
Reasonable Faith, by William Lane Craig
The Historical Reliability of the Gospels, by Craig Blomberg
The Case for Christ, by Lee Strobel."