Yes, secularism does propose a physical and purposeless universe, and many (but not all) of us accept the notion that our sense of self is a neuronal illusion. But although the universe is purposeless, our lives aren't. This conflation of a purposeless universe (i.e., one not created for a specific reason) with purposeless human lives is a trick that the faithful use to make atheism seem nihilistic and dark. But we make our own purposes, and they're real. Right now my purpose is to write this piece, and then I'll work on a book, and later I'll have dinner with a friend. Soon I'll go to Poland to visit more friends. Maybe later I'll read a nice book and learn something. Those are real purposes, not illusory purposes to which Douthat wants us to devote our only life.