That has not happened. Although mainline Protestantism is in decline, sectarian forms of religion are thriving (reports of their death have been greatly exaggerated -- "The Myth of Evangelical Decline"), not only in rural areas but also in cities. Moreover, religious beliefs and practices are on the rise and unlikely to disappear any time soon. Christianity is growing rapidly in the global South, Russia has experienced a religious revival, and Christians now outnumber communists in China. To be sure, Europe remains the poster child of secularization, but church attendance now isn't too much lower than it was in medieval Europe ("Is Secularization Inevitable?"). Then there is the 20% of Americans who report no religious affiliation; however, of those who say they have no religious affiliation, 18% consider themselves religious, 30% have had a religious or mystical experience, 33% believe that religion is somewhat or very important, 37% consider themselves spiritual but not religious, 41% pray weekly or more, and 68% believe in God. It may be difficult to categorize such individuals, but “irreligious” and “secular” they are not.
This, and much more, is the topic of the most recent Research on Religion podcast ("Rodney Stark on The Triumph of Faith"). It features the sociologist Rodney Stark, author of over 30 books and 100 academic articles. Stark not only discusses the religious situation in the United States (e.g., the apparent lack of religion among Millennials) but also in Europe, Africa, Latin America, China, Japan, and the Middle East. Here is a brief description of the interview (from the Research on Religion website):
The decline of religion around the world may be greatly exaggerated. Returning for his sixth appearance on our podcast, Prof. Rodney Stark, co-founder of Baylor’s Institute for Studies of Religion, discusses his new book “The Triumph of Faith” and reviews how the religious landscapes in various countries and regions of the world has been greatly transformed in the past half century. We look at “nones” from the United States, the rise of indigenous Christianity in Africa, and how even the Japanese still rely upon Shinto priests for blessings.
You can download the podcast from iTunes or listen to it at the Research on Religion website, where you can also find a more complete description of the podcast ("Rodney Stark on The Triumph of Faith"). You could also read Stark's book, which is entertaining, informative, and a little bit feisty (he's a little tired of fighting the "secularization theory" battle).