In the book, Hart outlines how Christianity transformed the ancient world in ways we may have forgotten: bringing liberation from fatalism, conferring great dignity on human beings, subverting the cruelest aspects of pagan society, and elevating charity above all virtues. He then argues that what we term the "Age of Reason" was in fact the beginning of the eclipse of reason's authority as a cultural value. Hart closes the book in the present, delineating the ominous consequences of the decline of Christendom in a culture that is built upon its moral and spiritual values.
That's a very good synopsis of the book, and probably more helpful than anything in my review. In any case, I definitely echo the high recommendation of the estimable Archbishop.