An essayist and critic of Haredi and Hasidic American Judaism, Deen is a regular columnist for The Forward and founding editor of Unpious, a magazine for voices critical of the Hasidic lifestyle and beliefs. He became known for his blog Hasidic Rebel, which he wrote from 2003 to 2012, which was the subject of a 2003 article in Village Voice. The blog was the first website written by a practicing Hasidic who expressed doubts about his religious faith. In 2010, he launched the Unpious website together with Shtreimel, another former Hasidic blogger, and has been its editor throughout. It was conceived as a platform for voices critical of the Hasidic and Haredi lifestyle and beliefs. His memoirs, Those Who Leave Don't Come Back (2015), chronicle his transition from a devoted member of the Skver sect to a secular Jew. Deen's book is considered one of the best exponents of the subgenre of ancient haredi memoirs. In addition to his regular column in The Forward, he publishes articles in prominent media such as New Republic, Salon and Tablet. The common denominator of his work is the critique of the haredi lifestyle and worldview. He is currently a board member of Step by Step, an organization located in New York City that helps Jews leaving the haredi world in their life transition.