On October 26, Zohran Mamdani held a rally at Forest Hills Stadium—a venue that sits right across the street from my apartment. It was hard not to despair as tens of thousands of New York’s most caffeinated and clinically unstable activists flooded my quiet, tree-lined Queens neighborhood like a swarm of earnest locusts. I decided to treat the ordeal as fieldwork. Stepping outside with what I told myself was a neutral disposition, iPhone Notes app at hand, I talked to his devotees, observed their rituals, and tried, against all odds, to understand what gospel this man was preaching that could...