The Bible has a number of places prophecizing that the birth of the Messiah would be preceded by a Messenger (Isaiah 40:3; Malachi 3:1). These prophecies are supposedly fulfilled in Matthew 3:1-3; 11:10; John 1:23; Luke 1:17.
This prophecy regarding the Messenger coming to announce the impending birth of the Messiah has a problem in that there are no independent accounts or evidence outside of the Bible that would either confirm or falsify it. The Bible is being used as the only source of confirmation. It is clearly completely impossible to use a prophecy that is only confirmed in the Bible to confirm the Bible. That’s what creationists are trying to do. The story of the Messenger given in Matthew, Luke and John may have been added simply because the authors of those books knew of the prophecy and added a “just-so” story to make it seem as though the prophecy came true. Matthew and Luke appear to have done something similar in order to make it appear that Jesus was born in Bethlehem in order to fulfill a prophecy. Why would they not potentially make up a story about Messenger if that would fulfill another prophecy?
Consider Sophocles’ play “Oedipus the King”. In that play an Oracle, someone able to predict the future for the Greeks, predicts that Oedipus will kill his father and marry his mother. As the play finishes, we find that these things really happened.
What are the odds that two such bizarre things should come to pass? They must be pretty long.
But, of course, the only accounts of either the prophecy or its fulfillment are in the play itself. What are the odds that a talented author would write a play such that it would fulfill a key prophecy? They’re surely 100%!
Similarly, some of the early Harry Potter books make prophecies that come true in the later books.
In fact, there are prophecies in the Harry Potter books that are confirmed outside of those books. For example, at the beginning of "Harry Potter and the Sorcerer's Stone", while observing the delivery of Harry Potter, Hogwarts professor Minerva McGonagall predicts, "...there will be books written about Harry -- every child in the world will know his name!" While it may be a bit of an exaggeration to say "every child in the world" has heard of Harry Potter, that prophecy has effectively come true. You would have a difficult time finding any child over the age of eight-years-old or so in an English-speaking country who hasn't heard of him.
Those fulfilled prophecies do not mean that the Harry Potter books are divinely inspired!
Effectively that’s the situation that we have in the Bible. Though different authors are generally involved because the New Testament often fulfills a prophecy from the Old Testament, it is still the case that both the prophecy and the fulfillment of that prophecy are in the same book. What are the odds that a talented author of the New Testament would write a story that fulfilled a key prophecy in the Old Testament? If the New Testament author was aware of that Old Testament prophecy, then the odds are probably close to 100%!
No prophecy that cannot be confirmed outside of the Bible would be given any credence by an objective observer.