This is a good introduction to many of the main arguments concerning theism and atheism. Smart and Haldane manage to cover most of the main issues while at the same time making original contributions to the debate. The book is especially helpful for giving clear account of the issues surrounding the "fine-tuning" argument due e.g., to Richard Swinburne.
As they mention in the Introduction, neither Smart nor Haldane is a specialist precisely in philosophy of religion--both are well known especially for contributions to philosophy of mind--and this affects the book's overall approach. Smart begins with a defense of physicalism (the view that only objects whose existence is required by physical theory exist), arguing that this position should be accepted on the methodological ground that it is most compatible with the results of modern science. Smart also responds to various theistic arguments and defends a version of the "problem of evil" objection to theism. Haldane follows this with a series of arguments against materialist reductionism, taking the failure of reductionism to entail some kind of design and so theism. Haldane also defends versions of the cosmological ("firt cause") argument and attempts a solution to the problem of evil. Smart then briefly responds to Haldane and Haldane to Smart. For Smart, atheism is part of a general commitment to physicalism, whereas Haldane seems no less interested in defending a general antireductionism (e.g., with respect to intentionality) than in defending theism specifically.
The book covers a great deal of ground and offers much food for thought. The downside to this is discussions of particular issues are sometimes sketchier than one would like. Perhaps it would have been more effective to focus the book more tightly on the aspect of the debate that raises issues of reductionism vs. antireductionism. I also wish the book had been organized so as encourage a more extended response from Smart to Haldane's antireductionist arguments. Smart's methodological principles may well establish a presumption against theism, but surely this presumption is defeasible, and Haldane's contribution is effect an attempt to defeat precisely this presumption.