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Because he starts off later in the Gospel with the same kind of stuff
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that you would see if you read a book about Augustus the Emperor,
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or Plato the great philosopher.
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That is, he starts off with narratives about a miraculous birth.
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Telling stories about a great man and his miraculous birth was
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a not uncommon way to start a life of someone.
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Then in Acts it also looks like he knows he's writing something
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that would look something like a history,
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and we have histories also from the ancient world.
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This guy is much more then Mark,
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this author is much more self conscious in setting himself forward
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and setting his work up as a literary work, it has literary form.