I think that Twain is pretty much satirizing everyone involved here. He is trying to point out how evil the king and duke are because they are going to scam people at a church meeting.
But he is also pointing out how the people who go to the revival are incredibly gullible and eager to try to reform people. This is a typically American impulse -- to try to save others who have gone astray.
Finally, he seems to be implying that the whole revival meeting is a scam by the people putting it on -- sort of like some televangelists seem to be today.