"That makes it impossible to know for certain what he wrote in every passage (or arguably, in any passage). "
As Ehrman points out, there are more textual errors and variations in the extant manuscripts than there are words in the NT. Even if god existed and did inspire the Bible, that fact alone (the number of errors) reveals his ineptitude, or perhaps outright apathy, in making sure the message was preserved intact.
All that is available are copies of copies of copies. Interestingly, professional, educated Christian scribes did not make an appearance until the 4th century. Their work was based on the existing manuscripts produced by uneducated, non-professional Christian "scribes" who scribbled copies of letters to be read to all the illiterate Christians (90% or more).
At best, what professional Christian scribes had as source material was the already hopelessly flawed material produced by amateurs.