Race
James Weldon Johnson was writing at a time when African-Americans were in the middle of their struggle for freedom. At the turn of the century, when Johnson lived, African-Americans still didn't have basic rights, like the right to vote or to attend desegregated schools.
For this reason, you'll find that a lot of his poetry deals with racial issues. "Lift Every Voice and Sing" is a poem about the struggle for racial equality. This theme is a big one, not only in this poem but also in others of Johnson's poems, including "To America" and "The Black Mammy."