Christianity depends largely on ancient documents, written by faithful men a long time ago. In response, skeptics often seek to undermine the reputation of the Bible, suggesting that it’s fictional, unreasonably biased, or unreliable. Christians should be able to explain why we consider these ancient documents the Word of God.
I hear it all the time from skeptics: the Bible didn't exist for hundreds of years after Jesus died. If true, that would imply that "established Christianity" differs from "original Christianity." This opens the door to all kinds of other nonsense, like Jesus not existing, Jesus not being God, and more.