How Do We Know the Protestant Canon of Scripture Is the Correct One?

Questions about how we know the Protestant canon of Scripture is the correct one, how we can know the New Testament books are inspired if we don’t know who the authors were, and how to talk to a friend who denies Paul’s apostleship and writings.

• How do we know the Protestant canon of Scripture containing 66 books is the correct one as opposed to the Catholic canon?
• If the authors of 20 of the 27 books of the New Testament are unknown, how do we know those books are inspired if nobody knows who wrote them?
• I have a friend who claims to be a Christian but who believes the earth is flat and denies Paul’s apostleship and writings. How should I discuss these matters with him?