No one (not even the best scholars) know with certainty who wrote the New Testament letters, just as we do not *know* who wrote the Gospels & the books of the Tanakh. All we have is “tradition” & lots of curators & editors, ranging roughly from the 500s BCE to 1500s CE. Also, said curators and editors are unnamed and/or unknown by Christianity and Judaism alike. The only non-Tanakh written record of David’s “House” is dated to 840 B.C.E. (See NOVA)
A case can be made that the Bible’s editing / redaction continued into the Reformation …and later, with every “version” or translation or “archaeological discovery.” For example, the “Dead Sea Scrolls” [DSS (found in 1947)] supposedly date to the 1st Century CE & support the accuracy of the Masoretic Text (7-10 C CE). But, the DSS were “preserved by minerals that cannot be found in the surrounding area – leading to questions over where & how the ancient text was produced.” (MIT via The Independent)
Convenient to the Urtext debate, “The text of the Dead Sea Scrolls …read somewhat in-between the Masoretic Text and the old Greek”… & the DSS were “randomly” found at WWII’s end, exactly when Israel was gaining statehood & before fighting its WoI (World Archaeology). Reportedly, the DSS were “written between 250 BC and AD 68: centuries that were pivotal in the history of both Judaism and Christianity. The scrolls pre-date all other known biblical writings by at least 1,000 years” (World Archaeology).
Most scholars “guesstimate” the copyists and editors of the DSS to be the Essenes, a 1st Century CE apocalyptic, isolationist sect (a.k.a. doomsday cult). But, again, the origins of the DSS are questioned, given that the origin of minerals used to preserve the documents is not local or regional.
NOW, let’s talk about the New Testament writings. There are no originals, despite thousands of copies & edited manuscripts. Do they agree? Mostly, they agree but not 100%, not even 99% or 98%, and when you’re dealing with a fundamentalists’ inspired text, that is kinda a big problem. Fundamentalist scholars will say “no essential doctrine is changed by the differences,” but Mark 16 is that one part of the Gospels, wherein Jesus (despite having previously condemned religious hypocrites for proselytizing) is attributed with saying in essence, ‘go preach everywhere to everyone, if they don’t believe they’ll be damned.’ That makes for an expressly fundamentalist evangelical foundation. But let’s back up.
There was no singular collection of “authorized” Christian texts until 325 CE, when Roman Emperor Constantine wanted to quash all the Xian sectarian infighting and establish a State Religion. He “invited” 1800 Xian Reps. Only 300 showed up & made a decision. Constantine commanded the 16% (hardly a quantum of those invited) to produce a list of the “received” (undisputed) texts that all Xian communities use. In other words, those obedient to Constantine told the emperor, ‘these are the texts we think people say are ok, due to common tradition.”
Regarding authorship, of the texts that were approved by the 325 CE Nicea Council, all of them were attributed to writers’ names used through almost 3 Centuries of tradition. And again, much of the infighting among sects at the time was about which texts are worthy. Imperialists decided. That list of books became the standard for Constantine’s Roman Xianity; & about 55 years later, Theodosius I decreed everyone not in Rome’s Xianity to be insane.
Roman Catholicism didn’t visit the matter again until the Reformation, when European kings, priests & translators made protest. Suddenly, in the face of their texts & practices being scrutinized, Rome decided to convene about what texts are canon (measure / standard) of “ancient Christianity.” Nicea was heavily relied upon, of course & “ancient” manuscripts sometimes appeared out of nowhere with fresh ink.