Looking at history, Grayling sees two major conficting views on the good: the humanistic and the theistic. He is by admission partisan, and views the humanistic conception of good, defined as pursuit of happiness so long as it does not intefere with the happiness of others, autonomy, independence, and the cultivation of the intellect, to be inherently superior to the theistic one, which Grayling sees as being about heteronomy, nihilism, and obediance to authority.
Where humanism premises autonomy as the basis of the good life, religion premises heteronomoy. In humanist ethics the individual is responsible for achieving the good as a free member of a community of free agents; in religious ethics he achieves the good by obediance to an authority that tells him what his goals are and how he should live. Given that the metaphysics of religion is man-made, and that human psychology is the source of belief in the power of transcendent authority to reward obediance or punish its opposite ('sin', one must remember, explicitly means disobediance) it follows that the chief motivation for religious ethics is the need felt by potentates of many kinds to exert control over individuals, to limit their freedom, to make them conform, obey, submit, follow where led, accept what is meted out to them, and resign themselves to their lot.What's more, humanists ground their conception of what is good in the reality and facts of the human condition, while theists ground theirs in a transcendental realm which is out of reach of human experience. It is Grayling's opinion that the religious conception of good is thus at odds with the pursuit of knowledge, and must by necessity tend to be reactionary and fundamentalist in nature. But unlike some militant atheists (take Sam Harris or Richard Dawkins, for instance) Grayling presents his views in a a non antagonistic and non confrontational manner.
If you have ever been at a loss to explain to someone what or why exactly secular humanists feel the way they do about religion, then this would be the book to read or recommend to others. If you are a believer then this book is also for you, as it offers an insight into the perspective of the humanist non-believer.