The books of the Bible were written well past anyone present could have still been alive. It’s the gospel “according to” Mark/Matthew/Luke. Not written and definitely not final edit by them.
By the point they were written down (~120CE), anyone around would have been dead at least 40-50 years. The cult of Christianity had split off from Judaism after the destruction of the temple and several claimed to be the true leaders. Various books were written and brought to bear, only some of those ended up being edited into the canonical (Catholic) books we know today.
The Ethiopian and Orthodox and other now extinct churches and what eventually developed into Islam also used other books (some more/less mystical and gnostic than others) and interpretations. There is also reference to many lost books in all the writings, even Jesus and Paul refers to the lost books eg the book of Enoch.