The author does not overestimate the importance of this little book: it is nothing more than its title claims. It consists of three lectures given to popular audiences, with the accompaniment of many illustrations. It represents, however, a considerable amount of work in an almost virgin field. It has involved hard journeys to remote mountain monasteries, and days and nights of conversation and inquiry with many monks and priests. It is not, however, a profound study nor an exhaustive presentation. It barely touches many a subject, which would alone furnish more material than could be treated in three such lectures. It but scratches the surface.