The Synoptic Gospels (Mathew, Mark, Luke) are in the order that early church thought they had been written. In fact it is most likely that Mark was written first. Mark was secretary to St. Peter in Rome and so it is reasonable to assume that what we have in Mark is the reminisces of St. Peter himself shortly before his martyrdom in 64 or 65 AD. John was written, at the latest by 90 AD and was the last Gospel.
In fact Galatians (I think) was the first written contribution to what would become the NT, in 49 AD. With the question mark of Hebrews (which is not by St. Paul) as the exception, all the Epistles were collected and were being disseminated among the early Christian churches by 70 AD.
Revelations (or Apocalypse) was written and being distributed before 110 AD.
So the Canon, with a few small exceptions, was complete within the first 100 years of the Christian Church. The next 100 or so years saw additions and exceptions but the Canon was prettty fixed before the first Church Council at Niceae in 325. It was confirmed at Hippo in 397 (I think).
Because Luther and the other Protestant ‘reformers’ cast doubt on several books of the OT and the NT, Holy Mother Church in Council at Trent felt it necessary to reaffirm what the Canon was but it was not changed or altered from what it had always been since before the time of St. Jerome.
