In the late 19th century and throughout most of the 20th century, the demographics of religion has changed a great deal.

Some historically Christian countries, particularly those in Europe, have experienced a significant decline in Christian religion, shown by declining recruitment for priesthoods and monasteries, fast-diminishing attendance at churches, synagogues, etc. Explanations for this effect include disillusionment with ideology following the ravages of World War II, the materialistic philosophical influence of science, Marxism and Humanism, and a reaction against the exclusivist claims and religious wars waged by many religious groups. This decline is apparently in parallel with increased prosperity and social well-being. It appears increasingly common for people to engage in far-ranging explorations, with many finding spiritual satisfaction outside of organized churches. This is a demographic group whose numbers are growing and whose future impact cannot be predicted.

In the United States, Latin America, and Sub-Saharan Africa, by contrast, studies show that Christianity is strong and growing stronger, and many believe those areas to have become the new "heart" of Christianity. Islam is currently the fastest growing religion, and is nearly universal in many states stretching from West Africa to Indonesia, and has grown in world influence in the West. Hinduism, Buddhism, and Shintoism remain nearly universal in the Far East, and have greatly influenced spirituality, particularly in the United States. Explanations for the growth of religion in these areas include disillusionment with the perceived failures of secular western ideologies to provide an ethical and moral framework. Believers point to perceived terrors such as Nazism, Communism, Colonialism, Secular Humanism, and Materialism, and the havoc wreaked by such movements around the world. Particularly vehement in this regard are Islamic fundamentalists, who view Western secularism as a serious threat to morality itself. They point to perceived decadence, high rates of divorce, crime, depression, and suicide as evidence of Western social decline, which they believe is caused by the abandonment of Faith by the West. These beliefs, however, fail to take into account objective evidence suggesting that cultural and social conditions, rather than religion, are responsible for the perception of marriage, suicide rates and crime in various societies. An example is the low divorce rate in the former USSR (a rigidly secular nation), and low crime rates in many predominantly secular European and Asia-Pacific communities.