The main thrust of the book seems to be that humanity is like any other animal and cannot control its own destiny. Scientific progress can give us longer life, but ultimately, because of our human nature, technology will destroy us. He maintains that humanism retains many secular Christian values, including the idea that man is special.
From the foreword:
Among contemporary philosophers it is a matter of pride to be ignorant of theology. As a result, the Christian origins of secular humanism are rarely understood. Yet they were perfectly clear to its founders. In the early 19th century the French Positivists Henri Saint-Simon and Auguste Comte invented the Religion of Humanity, a vision of a universal civilization based on science that is the prototype for the political religions of the 20th century. Through their impact on John Stuart Mill, they made liberalism the secular creed it is today. Through their deep influence on Karl Marx, they helped shape 'scientific socialism'. Ironically, for Saint-Simon and Comte were fierce critics of laissez-faire economics, they also inspired the late 20th century cult of the global free market. I have told this paradoxical and often farcical story in my book Al Qaeda and What It Means to be Modern.
Humanism is not science, but religion - the post-Christian faith that humans can make the world better than any in which they have so far lived. In pre-Christian Europe it was taken for granted that the future would be like the past. Knowledge and invention might advance, but ethics would remain much the same. History was a series of cycles with no overall meaning.
Against this pagan view, Christians understood history as a story of sin and redemption. Humanism is the transformation of this Christian doctrine of salvation into a project of universal human emancipation. The idea of progress is a secular version of the Christian belief in providence. That is why among ancient pagans it was unknown.
Belief in progress has another source. In science, the growth of knowledge is cumulative. But human life as a whole is not a cumulative activity; what is gained in one generation may be lost in the next. In science, knowledge is an unmixed good; in ethics and politics it is bad as well as good. Science increases human power - and magnifies the flaws in human nature. It enables us to live longer and have higher living standards than in the past. At the same time it allows us to wreak destruction - on each other and the earth - on a larger scale than ever before.
I'm not sure where he goes with all this, and from what I've seen so far, he makes claims but doesn't always back them up. The book does seem like interesting food for thought.