When Mencken returns to Dayton from the revival, he seems struck by the difference between what he's just seen, with people speaking in tongues and expressing what he views as simple, primitive, fanatic zeal for their religion, and the ludicrous scene in Dayton, with people from all around selling Bibles, giving fiery speeches, and advertising the end of the world.
"The note of devotion," Mencken says, "was not there" in Dayton. People came to see the events of the trial almost as a curiosity. While he's not at all sympathetic to the worshippers in the "Hills of Zion"- in fact, he explicitly says he'll leave their actions to be interpreted by Freud-he finds them far more interesting than the "friars wearing sandwich signs" and Judge Raulston "judiciously picking his teeth" at the hotel. He's stunned by what appears to him to be the incredible ignorance of Tennesseeans in both gatherings, but at least the revivalists are sincere, in his view.
From the tag on this question, I assume that you are asking about H.L. Mencken's column "Among the Believers" that is included in many anthologies of documents for history students. In that column, Mencken is arguing that the people of Dayton itself are not really all that religious. The truly religious, he says, are the people from back up in the hills. The party atmosphere in town, then, is different from the revival meeting because there is no true belief there -- it is more like a circus than it is like the revival meeting.
Mencken clearly thinks that the hillbillies at the revival meeting (and other true believers like the woman who thought Coca-Cola was from the devil) are ignorant and backwards. He cites the example of the woman who thought that reading books was wrong. But he does not doubt that they truly believe.
By contrast, Mencken says, the people in Dayton see the theology as a form of entertainment. They listen for a while, he says, and they go off to get a drink. The "real religion," Mencken says, is not present in Dayton -- it is only off in the hills.