The manuscripts were produced after the fact, yes, but they were produced by those who experienced the times or who were related to or well acquainted with those who had experienced the times.
That's the story we have, and yet, we don't have any original versions and the authorship of many of the books is questionable. Further, there were clearly disparate perceptions of "the truth"...some were labeled as heresy and attacked, others were labeled as "orthodox" and enhanced. With an oral tradition, maintaining orthodoxy is considerable harder. We don't know for sure which of what are now accepted doctrines were really heresy or orthodoxy, because that distinction resided primarily in the subjective judgement of the original apostles, and moreso...in the subjective judgement of the people who were instructed by those apostles and then dispersed by persecution.
.And we know an enormous amount about why the texts that made it into the Bible were chosen, how they were chosen, and the debates that went into their selection
Except we don't have the actual list of books dating from 363. Odd, eh? And these decisions weren't made until the mid 4th century, when the faith could already have diverged significantly from the original message.
Mysteries abound within them, yes, and I am reasonably certain they were intentional in the face of the persecutions Chrsitians were receiving from both the orthodox Jews and the Romans. The Apostle Paul himself started out obsessed with a personal goal of stamping out all Christians who, at that time, were considered to be all heretical Jews. Otherwise he wouldn't have cared.
The apostle Paul was also obsessed with putting his own doctrinal stamp on the early beliefs, and largely succeeded, even though Peter was appointed by Christ to lead the early Church.
There are components of my religious beliefs that are based on faith, yes. But the history of the growing Church, the debates that went into the theology that was ultimately declared 'orthodox', and why the particular manuscripts were chosen for the Old and New Testaments are all a matter of record, not faith. It does require an extensive amount of rather tedious study to get to the heart of it all.
After the mid 4th century, you're correct. But, from the first century, we have very little that is original and authenticated.
We have several of Paul's letters--some in their entirety and pieces of others--plus others that were more likely written by those he mentored than likely written by him. The oral tradition was well practiced among orthodox Jews who were very skilled at it, and the manuscripts were written well within a time frame in which no legend or prominent mythology had time to develop.
There are as many dedicated conservative church historians and theologians who have arrived at much different conclusions than some of the more modern liberal seminaries are teaching. Those historians and theologians have no ax to grind, no particular political or heirarchal perspective to defend, and have done yeoman's work in giving us as much truth as was available at the time.
None of them, for instance, believe the Synoptic Gospels were written down by any one individual but rather are collections of manuscripts collected and edited together. This in no way detracts from what we can learn from those manuscripts.
And in the final analysis it really isn't important who wrote them when God is perfectly capable of speaking through them. As a matter of faith, I think we don't have the original manuscripts because if we did, we would worship them instead of the One those manuscripts address. If you believe in the God of the Bible, you know He is still speaking to us. If you do not, then it really becomes absurd to debate the scriptures and what they mean at all isn't it?