A church in Arizona sits along the U.S.-Mexico border. (Photo: EthicsDaily.com)
Whatever might drive them to move, migrants carry their religions with them.
Yet the religions they bring to their new lands do not remain unchanged.
The fact of movement itself is a powerful dynamic force in religious change, and this is nowhere more obvious than in the United States.
In his classic book, "The Uprooted" (1951), Oscar Handlin wrote, "Once I thought to write a history of the immigrants in America. Then I discovered that the immigrants were American history."
I am tempted to offer an adaptation. "Once I thought to write a history of immigrant religion in America. Then I discovered that the immigrants were American religion."
Migration has two critical effects on religious practice:
1. It encourages people to form new communities, which tend to be religious in character.
2. It severs them from their homelands, which often had a special spiritual significance.
Taken together, these factors stir both religiosity and religious innovation.
Movement and migration cut individuals off from their homes and familiar social networks, driving them to seek new networks and forms of instant community.
Commonly, the most important such institutions historically have been religious, whether churches, synagogues or mosques. Believers find here opportunities for fellowship and community, but also (often) the basic necessities of welfare, education and health.
Immigrants participated heavily in their churches, not necessarily because of fervent piety, but because those institutions provided the essential social, cultural and economic framework that allowed displaced communities to survive in a puzzling and often hostile land.
This is where they could speak languages from home and find familiar food, where children could be taught and where young adults find marriage partners (a major consideration).
The harsher and more hostile the mainstream environment, the greater the appeal of the religious community.
Through American history, successive waves of immigration have produced many new religious institutions, which, at least for the early generations, are closely linked to national or ethnic loyalties.
In most cases, involvement in migrant churches and institutions produces a much greater degree of active religiosity than was common in the home country.
This was true of Italian Catholic migrants to the U.S. during the early 20th century, who came from societies with a lively streak of anti-clericalism, and in which religious life was often assumed to be a female preserve. On American soil, however, Catholic practice and identity both grew more intense.
Among modern Latino immigrants likewise, the journey from Central America to the U.S. produces much greater religious interest and involvement than is customary at home, and the new enthusiasm resonates through both Catholic and Protestant congregations.
Migrants seek to make religious institutions a piece of home. But just suppose that, as Thomas Wolfe said, "You can't go home again"?
For many, the experience of migration produces religious demands that simply cannot be met within the older religious framework.
The religion of traditional societies is often intimately linked to particular places, to shrines and sacred sites, and moving to a new country obviously cuts those ties.
People abandon the old sacred landscapes they knew in their rural homes, with all their saints and shrines, and a sacred year marked by religious feasts and fasts.
In the cities, instead, they adopt a globalized form of modern faith, characterized by sophisticated modern media and advertising, including the most contemporary social media. Commonly, that means Pentecostal forms of worship.
But whatever new believers have lost through cultural change, they feel that they have gained much. However poor in material things, they know in their hearts that they are following a pristine form of apostolic faith.
Nor is that kind of break unique to Christians. As French scholar Olivier Roy shows, such a pattern goes far toward explaining contemporary trends in Islam.
Roy relates global religious change to such megatrends as mass migration, urbanization and modernization.
He stresses how deeply integrated Islam was in traditional societies like Morocco or Pakistan, where faith was tied to particular communities and clan structures, to shrines, saints and sacred landscapes, and to a sacred calendar.
All were severed with the migration to the West, creating an Islam that was suddenly and painfully de-territorialized.
While early Muslim immigrants kept their personal memories alive, none of these traditions was available to younger generations born in Europe.
Like their Christian counterparts, they lost their roots, all moorings of sacred time and place.
As Roy remarks, "The religion of their parents is linked to a culture that is no longer theirs."
Yet while losing traditional culture and faith, they find little appeal in the materialist values offered by their new host countries. They responded by turning to new forms of global Islam, often in fundamentalist guise.
Scholars often debate the origins of America's legendary religiosity, which marks it off so sharply from comparably wealthy and advanced Europe.
I believe that the country's constant pattern of mobility and migration over long distances is a major part of the story, and one that sets the country apart from Europe.
Americans are ever being uprooted and replanted. A moving and restless people is one that is ever in need of stable institutions that provide community and a sense of place and tradition, and it is usually the churches and synagogues that have filled this role.
And now, America's mosques and temples.
Philip Jenkins is distinguished professor of history at Baylor University in Waco, Texas, and serves as co-director for the program on historical studies of religion in the Institute for Studies of Religion. He is the author of numerous books, including "The Great and Holy War: How WWI Became a Religious Crusade." A version of this article first appeared on The Anxious Bench, where he blogs regularly, and is used with permission.