The author of ‘The World’s Religions’ looks back on 90 years of prayer, yoga and dropping acid.

Huston Smith is doing publicity for his 14th book, though at nearly 90 he is debilitated by osteoporosis and can’t get up from his leather chair. When a visitor enters his room at the Berkshire, an assisted-living facility in Berkeley, Calif., he smiles broadly, indicates a small wicker seat and cheerfully warns of a high-pitched whine that sometimes emanates from his hearing aid. One is reminded that there is nothing glamorous about growing old, but Smith—who has arguably been the most important figure in the study of religion over the past five decades—makes it look at least bearable.