The only way to conquer Broadway’s annual post-Labor Day slump, it seems, is to have a little faith.Faith in “The Book of Mormon,” anyway: In a week when almost every single show on the Main Stem saw sales drop, “Mormon” ($1,293,582) rose — and managed to top perennial smash “Wicked” ($1,253,990) in the process and almost nick “The Lion King” ($1,299,169). The “Mormon” feat is especially impressive given that its theater seats only about 1,075 vs. 1,800 at “Wicked” and 1,675 at “The Lion King.” Otherwise, it was a tough week all around, with major slides posted at most of the shows on the boards. “Lion King,” “Wicked” and “Spider-Man: Turn Off the Dark” ($1,197,278), for instance, all saw week-to-week sales drop by about 20%. The yearly slowdown is attributable to the passing of Labor Day, the official demarcation of the end of the summer travel months. But this year the effects were exacerbated by the visitor attention directed to the city activities organized around the 10th anniversary of 9/11. Productions with two shows skedded during that Sunday (as opposed to only one at most shows on the Rialto) were likely hit the hardest, as was the case with, for instance, “Mary Poppins” ($492,889), down a whopping 35%. Plus, with the concurrent startup of the school year, a number of productions with family appeal — including “Poppins” and Daniel Radcliffe topliner “How to Succeed in Business Without Really Trying” (down by almost $300,000 to $757,240) — took some of the hardest hits of the frame. Some of the more robust newer offerings from last season slipped, but less so: Both “War Horse” ($943,131) and “Anything Goes” ($732,228) downticked only a bit, especially compared to the rest of the Street. Buzz-challenged “Sister Act” ($463,035), on the other hand, played to average auds of less than 50% of capacity. Meanwhile, tuner “Follies” ($743,353), which opened Monday night, held up pretty well given the sesh’s overall trend and the fact the musical accommodated a string of comp-heavy press perfs over the weekend. One show, “Hair” ($368,039), exited posting the same low sales as it had logged throughout its return run on the Rialto, while the week’s only new offering, Frank Langella starrer “Man and Boy” ($98,634) at the Roundabout, got off to a slow start. Overall Broadway cume fell $4.1 million to $14.3 million for 21 shows on the boards, with attendance falling to 165,000. In the silver lining department: That B.O. figure actually marked an improvement over the week ending Sept. 12, 2010, when 23 shows pulled in a total of $13.4 million.