It’s Tuesday night Bible study, which at this Crystal City Mormon chapel looks a bit like a mixer. Pews are filled with people in jeans and flip-flops, many texting. The night begins with a prayer before segueing quickly into an ice-breaker trivia game and a pizza social. Not surprising for a church made up exclusively of singles — 800 of them. The newly opened “23rd Street chapel” is unique in the American Mormon church: It is the only worship space in the country devoted solely to unmarried people in their 20s, 30s and 40s.