Photo: Adam Dowling for New York Magazine On a rainy Sunday evening in May, the mood inside Brooklyn Steel was, like the candidate, jubilant and young. Around 1,500 people had gathered for Zohran Mamdani’s campaign rally for mayor of New York City. This was a congregation of true believers, the evangelists who not only plan to vote for the 33-year-old democratic socialist and three-term Queens assemblyman but are trying to convince everyone else to do so. The evening’s roster was a