You mentioned in your series How the Bible Came To Be that apostolicity is the key to inclusion of a text in the NT canon. How do I know any writing is apostolic, since I cannot say for certain if any of the works attributed to the apostles were actually written by them? The reference point itself becomes dynamic. How do I answer the skeptic? — V.B.
"Apostolic" simply means that a teaching matches the teaching of Jesus Christ and his apostles. It fits like a hand in a glove. See also Hebrews 2:1-3.
As for authorship, we know Paul wrote his letters. These aren't anonymous. In fact few of the NT letters are anonymous! The four gospels are anonymously written, but there are strong and early historical reasons for traditional authorship. Besides, Mark is with Peter; Luke is with Paul; so the non-apostles among the four evangelists (Mark and Luke) are still apostolically connected.
The early church was well positioned to determine whether a document resonated with the authentic apostolic or dominical (spoken by the Lord) message—certainly better positioned than we are.
(You are right, however: things did eventually become more dynamic—when the church of later centuries began to claim authority equal with that of the apostles, and created numerous doctrines, regulations, and dogmatic texts that nullified the original apostolic message.)