Being a silent film, The Birth of a Nation isn't exactly known for its screenplay. There are really not a lot of memorable lines in this one.
The script was written by D.W. Griffith and Frank E. Woods, and is primarily based on the book The Clansman by Thomas Dixon Jr., which had been adapted into a play ten years prior. We say primarily because the movie also pulls from a range of "historical" sources as well. The main bit from The Clansman is the story arc that composes the second half of the film, in which the Cameron family fight the forces of Silas Young.
Interestingly, there are reports that Griffith directed most of the film from memory rather than consulting a script, which is a seriously impressive feat. Three hours is a lot of movie to rattle off the dome.
Of course, the actual content of those three hours might be ridiculously offensive, but it's still another example of Griffith's filmmaking prowess.