Whitman couldn't possibly be more blunt in saying that he does not exclude anyone.
(He would make a bad basketball coach: he'd never have the heart to cut anyone.)
He's having a party, and everyone is invited. Look, it's the thief! And the slave! And the guy with an STD! Come on in!
The reference to a fateful meal is telling. Whitman seems to be channeling the passage from the King James Bible about the Last Supper, Jesus' last meal with his disciples where he says, "This is my body which is given for you." Whitman repeats the phrase, "This is" several times throughout the passage.
Is Whitman trying to take the place of Jesus Christ? What do you think?
He claims that his purpose is the same as nature's purpose.
Finally, Whitman is going to single out someone for special privileges: us! He's going to tell us a secret that he "might not tell everybody." (Hmm. But isn't he telling everybody the same thing right now? Tricky!)