Perhaps it is your reading comprehension that is the problem.
They were circulated and read to the early churches far earlier than 300 AD.
Again, I will tell you that I understand that the letters were all written before 100 AD, most probably before 60 AD, depending upon your take on Johns letters to the 7 churches and Christs Revelation.
However, the vast majority of the Christians of the day never had the privilege of hearing them read. The letters were sacred, and very important. The Christian leaders kept them secret, and the Roman's wanted them destroyed.
If you were caught with one it was a death sentence for you. So it is not like there were millions of copies floating around the Empire. The ones that existed were mostly copies because the originals were destroyed or just withered away in time due to the way the parchments were made.
But I am finished with going back and forth with you over this.
Why cannot more of you give an answer to the question like
LetMarch did, instead of taking me or the author to task for daring to ask such a question?