The American constitution was amended after the Civil War, but in the original version, not only was slavery legal, but a slave was counted as precisely three-fifths of a person. So Frederick Douglass puts a lot of time and energy into arguing that slaves are Americans too. In fact, when he mentions American hero Patrick Henry, he's even making the case that the slaves who fight for their freedom are at least as American as founding fathers like Henry, since they're just as willing to die for their freedom as he was.
By comparing escaped slaves to Patrick Henry, Douglass suggests that slaves fighting for their freedom are the true patriots.
Since Douglass believes that a slaveholding nation cannot truly live up to the ideals of the Constitution, he believes that the South is unfaithful to the American spirit.