Martin specializes in the philosophy of religion, though he has also worked on the philosophies of science, law, and social science. He is the author or editor of a number of books, including Atheism: A Philosophical Justification (1989), The Case Against Christianity (1991), Atheism, Morality, and Meaning (2002), The Impossibility of God (2003), The Improbability of God (2006), and The Cambridge Companion to Atheism (2006). He sits on the editorial board of the philosophy journal, Philo.
In his Atheism: a Philosophical Justification, Martin cites a general absence of an atheistic response to contemporary work in philosophy of religion, and accepts the responsibility of a rigorous defense of nonbelief as his "cross to bear:"
The aim of this book is not to make atheism a popular belief or even to overcome its invisibility. My object is not utopian. It is merely to provide good reasons for being an atheist. … My object is to show that atheism is a rational position and that belief in God is not. I am quite aware that atheistic beliefs are not always based on reason. My claim is that they should be.