"What to the Slave is the Fourth of July?" is a persuasive speech, but it is also structured according to a particular form: the Jeremiad. A Jeremiad is a long prose text that laments the current state of society, usually with a polemical tone, and ultimately predicts the imminent downfall of that society. The form is named after the biblical prophet Jeremiah and the books of the Bible he is credited with composing: Jeremiah and Lamentations. In Jeremiah, the speaker predicts the destruction of the Kingdom of Judah after the people's broken covenant with God.
The Jeremiad form is also closely associated with American Puritanism and the early colonists of New England in the seventeenth century. It has roots in the notion of American Exceptionalism, a colonial philosophy that maintains that the United States is fundamentally different (and usually superior) to other nations. When the early English settlers established the first American colonies, they honed this idea that the country they were founding would diverge from their original home in England. As such, the Jeremiad form developed as a way to criticize England and other imperial countries while bolstering the nascent America as a novel representation of what a nation could be.
That Douglass chooses the Jeremiad form is significant, then, because it both further showcases his intimate knowledge of the Christian Bible (which aids his argument against the Church in the latter half of his speech) and provides another instance of profound irony on which the majority of the speech relies. With the Jeremiad having a connection to American colonization and American exceptionalism, Douglass's choice to use that very form to lament the state of America is in keeping with his broader argument that liberty in America is nothing but a myth. The form of Douglass's speech essentially turns American history against itself, dramatizing how the ideals and values that American citizens celebrate are meaningless if those founding principles are not also extended to those currently enslaved.