Which implicit element of the analogy in these lines is analogous to "family worship"?

So the implicit (meaning implied, not directly stated) analogy here is "building." The author, speaking through the narrator, is likening Aunt Polly's daily family worship to the building of a structure from which she could deliver her strict sermons. She's not actually building, of course, but by starting with basic quotations from the Bible, which she would then use her own words to link together, she could then "stand" on that foundation and impart the "grim" (scary, 'if you misbehave you'll be doomed to go to that bad place below') type of stories.