<span>"Five score years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation. "
In the second paragraph he is referring to Lincoln's Emancipation Proclamation which was made a hundred years before King's speech, "I Have a Dream." Lincoln was a huge contributor to the fight against discrimination.
An allusion is a literary device where the speaker or the author refers directly or indirectly to a well-known person, place, event or literary work. In this case it was used to contribute to the message of what he is conveying.
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Catholicism has always been a huge religion, and has been in control of the world for previous ages. For that reason, they were the ones who could dictate when to start the timeline, and chose the birth of Christ as that starting point.
