One of the most powerful elements of Douglass' writing is how he raises the imminent critique or contradiction between a nation predicated upon human freedom and one that allows slavery to exist. Douglass is quite pointed in suggesting that the reality of America is denied when it permits slavery. Essentially, Douglass is forcing the issue with the contradiction between America's promises and its reality. In the process, Douglass reveals a great deal about the hypocrisy of the founding fathers and those who have inherited the positions of power in American government and society. For Douglass, until the nation can effectively outlaw and stop slavery, it will live in the chasm between its hopes and its actual function.
In 1852, while living in Rochester, NY, Douglass, a former slave turned editor and public abolitionist speaker, was asked to speak for a fourth of July celebration. Instead of delivering a speech glorifying and celebrating the nation's independence, he delivered a massive attack against a country that violates its own declaration of independence by allowing so many people to remain enslaved. He poses a key question as to whether or not the rights are given to all:
Are the great principles of political freedom and of natural justice, embodied in that Declaration of Independence, extended to us?
It is clear that they are not given to all, and Douglass sees and calls America out on the hypocrisy of these words. He notes that the founders crafted a document to afford equal protection and rights to all when they drafted the constitution, but those rights are not actually extended to all human beings. Slavery, as long as it exists, nullifies the declaration of independence as a a statement of rights extended to all.
I assume that you are referring to the speech that Douglass gave on Independence Day of 1841 in Rochester, New York.
In that speech, Douglass was saying that there was no reason for slaves or for black people in general to celebrate that day. He was saying that all the day did was to rub in to black people how hypocritical the US was.
Douglass argues that there is no reason for him to celebrate a document or the people who wrote it when the document and the people kept his people enslaved.