From what I understand, there’s a lot of extant copies of the New Testament from around 200-300 AD, more so than other contemporary Roman historical documents. So you can compare the different versions and mostly figure out what’s original and what was added later, but the real questions are what changed between 33 AD and 200 AD, and how do you translate them from the original languages (different books of the New Testament were written in Greek, Latin, and Aramaic).

The Old Testament is another story, translating ancient Hebrew involves a lot more guessing.

I’m by no means a biblical scholar, but there’s a spectrum of belief from “these are some ancient stories that might have some bearing on modern life because humans haven’t changed that much” to “these are the exact words of God handed down completely unchanged to King James and if you change one ‘thou’ you’re going to hell.” For the second option you have to completely ignore huge swaths of history and scholarship.