Unlike traditional Protestant and Catholic churches, whose memberships are dwindling, pentecostalism, the most experiential branch of Christianity, has become the fastest-growing form of worship on Earth, and, if present trends continue, could surpass even Catholicism by the turn of the century. From its obscure beginnings a hundred years ago in a tiny black church on Azusa Street in Los Angeles it has grown, especially in the past two decades, to over 410 million people worldwide, becoming the dominant expression of Christian worship in mega-cities throughout the world, from New York and Mexico City to Seoul, Korea. In an effort to understand its extraordinary appeal Harvey Cox has traveled the world to speak and worship with pentecostal congregations on four continents, and he has come to the conclusion that this explosion of spirituality represents a tidal change in what religion itself is and what it means to people.