An allusion is a brief reference to something the audience is familiar with that allows the speaker to bring up all the related thoughts and feelings that go along with that reference. You know how watching one episode of Stranger Things makes you think of The Goonies, E.T., and Stand by Me all at once? Yeah, that's the power of allusion at work.

And Douglass knows about the power of allusion.

Early in the speech, Douglass compares the United States to Babylon, which was felled by God because of its sins, according to the Bible:

Do you mean, citizens, to mock me, by asking me to speak today? If so, there is a parallel to your conduct. And let me warn you, that it is dangerous to copy the example of a nation (Babylon) whose crimes, towering up to heaven, were thrown down by the breath of the Almighty, burying that nation in irrecoverable ruin. (22-24)

Douglass also alludes to the Declaration of Independence (3), the Constitution (33), and the Bible (33), knowing all three of these to be sacred documents to his listeners. By saying these documents stand on his side, he strengthens his argument.