Whitman takes pity on the priests who have indirectly taken a beating in this poem. He says he doesn't hate them. But that doesn't mean he likes them.
His own personal faith is different from the world's religions. It is both "the greatest" and "the least."
His faith is the greatest because it combines all the other faiths into one. He views himself as containing the spiritual feeling of all the world's religions, which is part of his project of identifying with everyone and everything.
His faith is the least because he's a non-conformist and can't actually practice all of these religions that he describes. The speaker mentions numerous religions from the worship of Greek oracles to tribal shamanism to pagan worship of the sun to Hinduism, Buddhism, Islam, Christianity, Puritanism, and many more that are described but not named in this section.
He talks with the followers of these religions like someone leaving instructions ("charges") before setting off on a journey of his own.
He also identifies with the people who doubt – the atheists, skeptics, and pessimists. He compares them to flat fish called "flukes" that flail about when they are caught by fishermen.
He says he has no idea what comes after death, but he knows it is "alive."
Death will not fail anyone, from the young to the old, humans and animals.