I think that Whitman was something of a mystic in that he saw all of us as part of the whole; but it was the democratic whole. He saw himself as the poet that Emerson called for; Emerson saw him as the poet who answered his challenge.
This yields two themes: the unity of all being (and, if you read his endless lists of "things," none of which are subjugated to another but all treated as equal you understand where this is coming from. The other is American as the fulfillment of the democratic idea. This is similar to his mystical view of the unity of all things, but is more political, more based in the reality of what he saw and lived with.
Indians, blacks, Irish, gays, straighs, men, prostitutes, laborers, women, old, young ... these were all part of the theoretical unity of being, expressed in the structure of America.
Walt Whitman wrote of the divinity of the self (the "I" in his poems has been reborn as something divine) and of the individual as well as the community of man as in his "Leaves of Grass." Whitman was his speaker in "Leaves of Grass":
I am the poet of the Body and I am the poet of the Soul...
I am the poet of the woman the same as the man...
I am not the poet of goodness only, I do not decline to be the poet fo wickedness also ("Leave of Grass").
He also had themes of exploration and exaltation of sexuality, themes which brought much criticism upon him. Nature and its beauty are among themes treated by his poetry as are the melancholy and horrors of death in the Civil War. He received high acclaim for his elegy written after the slaying of Abraham Lincoln: "O Captain! My Captain!"