The influential writer known for popularizing the concept of existentialism is Jean-Paul Sartre. Sartre’s works, including "Being and Nothingness," explored themes of freedom, absurdity, and the conditions of human existence. His philosophy emphasized the importance of individual choice and responsibility in a seemingly indifferent universe, challenging readers to confront their own existence and the meaning of life. Sartre’s ideas not only established him as a key figure in existential thought but also influenced literature, philosophy, and the arts in the 20th century.