Upvote:12
I'll post an answer to one aspect of your question "How do we know that the Bible hasn't changed since it was written?"
It all boils down to manuscripts.
The number and age of the manuscripts that we have shows that nothing of significance has changed since the earliest Christian times.
Here is a list of some of the earliest manuscripts. For reference on this site:
Author Date Written Earliest Copy Time Span Copies (extent)
Magdalene Ms (Matthew 26) 1st century 50-60 AD co-existant (?)
John Rylands (John) 90 AD 130 AD 40 years
Bodmer Papyrus II (John) 90 AD 150-200 AD 60-110 years
Chester Beatty Papyri (N.T.) 1st century 200 AD 150 years
Diatessaron by Tatian (Gospels) 1st century 200 AD 150 years
Codex Vaticanus (Bible) 1st century 325-350 AD 275-300 years
Codex Sinaiticus (Bible) 1st century 350 AD 300 years
Codex Alexandrinus (Bible) 1st century 400 AD 350 years
Through these (and many other) manuscripts, we have been able to show that the Bible of today has not been altered at since its earliest manifestation.