Your friend saw this resource on bethinking.org and thought you should see it:
How Do the Gospels Compare with Other Ancient Documents?
Can we be confident that we have the original text of the Gospels? How does it compare to other ancient documents?
See the full resource online by visiting bethinking.org:
https://www.bethinking.org/jesus/gospels-compare
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bethinking.org is a collection of resources which aims to make sense of the big questions of life about God, Jesus and Christianity.