The decline of Christianity in the Western world is the decreasing Christian affiliation in the Western world. While most countries in the Western world were historically almost exclusively Christian, the post-World War II era has seen developed countries with modern, secular educational facilities shifting towards post-Christian, secular, globalized, multicultural and multifaith societies. While Christianity is currently the predominant religion in Latin America, Europe, Canada and the United States, the religion is declining in many of these areas, particularly in Western Europe, North America, and Oceania. A decline in Christianity among countries in Latin America's Southern Cone has also contributed to a rise in irreligion in Latin America. In the West, since at least the mid-twentieth century there has been a gradual decline in adherence to established Christianity. In a process described as secularization, "unchurched spirituality" is gaining more prominence over organized religion.

Background
According to a 2012 Pew Research Center survey, Christianity will continue to be the world's largest religion throughout the next four decades. However, Christianity may experience the largest net losses in terms of religious conversion, according to expectations. Worldwide, religious conversions are projected to have a "modest impact on changes in the Christian population" between 2010 and 2050; and may negatively affect the growth of Christian population and its share of the world's populations "slightly".However, these forecasts lack reliable data on religious conversion in China, but according to media reports and expert assessments, it is possible that the rapid growth of Christianity in China may maintain, or even increase, the current numerical advantage of Christianity as the largest religion in the world. In the United States, there have been some conversions to Christianity among those who grew up non-religious but it has not been in numbers that make up for those who were raised as Christians but became religiously unaffiliated later in their lives. Scholars have proposed that Church institutions decline in power and prominence in most industrialized societies, except in cases in which religion serves some function in society beyond merely regulating the relationship between individuals and God. Developing countries in Latin America and Africa are not experiencing a decline, mostly because of religious conversion in those countries where the Church offers broad social support services.Together with the decline of Western Christians, increasing numbers of Christians in the global South will form a "new Christendom" in which the majority of the world's Christian population will be found in the South. According to various scholars and sources, Pentecostalism – a Protestant Christian movement – is the fastest growing religion in the world; this growth is primarily due to religious conversion.