In 2015, the Pew Research Center released the largest study of American religious identity ever done in the United States of America, called "America's Changing Religious Landscape." The big discovery was that the number of American Christians had declined by 7.8 percent since the previous survey in 2007, while the number of Americans religiously unaffiliated had increased by 6.7 percent to 22.8 percent of the national population.

The only bright spot for Christianity was that, even though evangelical Christianity had declined as a percentage of the national population (down 0.9 percent), it had grown in real numbers by 2.4 million adherents.

There was a second important trend buried in the numbers and completely missed at the time. Hispanic immigration has propped up this declining American Christianity. The loss of Christian adherents would have been worse if it had not been for Hispanic immigration.

Evangelical Christianity owed its increase largely to Hispanic converts from Catholicism, and both mainstream Christianity and Catholicism would have sustained bigger losses if it had not been for increases in Hispanic membership. These trends indicate that, if the USA builds an effective border wall, immigration will further decline and, along with it, the number of Americans identifying as Christian.

The first trend is that the three largest types of Christianity—evangelical and mainstream Protestantism, and Catholicism—had big losses between the 2007 and the 2014 surveys. Evangelicals lost 8.4 percent, mainstream Protestants lost 10.4 percent, and Catholics lost 12.9 percent of their members.

Luckily, this was countered by newcomers joining these faiths. There were 9.8 percent new evangelicals, 6.1 percent new mainstream Protestants, but just 2 percent new Catholics. So, the final, overall numbers came out to a 1.5 percent increase for evangelicals, but a loss of 4.3 percent and 10.9 percent for mainstream Protestants and Catholics, respectively. The Pew report emphasized that many of these leavers left formal religious membership altogether. However, a significant number simply joined other types of Christianity.

The second trend is that none of the three types of Christianity can sustain its numbers by generational replacement. That is, the children of these adherents are fewer than the number of adults. This is largely because of declining family size. For example, 30 percent of evangelicals were born in the generation between 1928 and 1946, while just 21 percent were born in the millennial generation between 1981 and 1996. As the older generation passes away and the younger generation ages, the total number of adherents will drop. The imbalance is similar for the other two forms of Christianity. Without an influx of new blood, all three types of Christianity will decrease in size.

The third trend is that nearly 80 percent of the replacements for the losses in these types of Christianity come from Hispanics. All three have had a 5 percent increase in ethnic diversity in the seven years between 2007 and 2014. That increase is 80 percent Hispanic for all three forms of Christianity. Indeed, for Evangelicalism, the increase in the percentage of Hispanics accounts for its increase in real numbers of members. If Hispanic immigrants or their children had not joined these three types of Christianity, all three would have shown significant losses. Catholics would have dropped 12.5 percent of its members in those seven years (instead of 10.9 percent). Mainstream Protestants would have lost 9.2 percent (instead of just 4.3 percent), and evangelical Protestants would have lost 6.4 percent (instead of gaining 1.5 percent).

The fourth trend is that this Hispanic increase came in different ways. Most Hispanic immigrants arrive as Catholics. Catholicism's 5 percent increase is part of the decades-long influx of Hispanic members that has kept the numbers of American Catholics fairly steady. It also has helped Catholicism to become the most ethnically diverse form of Christianity: 41 percent of Catholics are nonwhite, and most nonwhites are Hispanic.

For the two forms of Protestantism, however, Hispanic membership has come from conversion. These are Hispanics who have been in America for a long(er) time, and they convert to this still-dominant form of American religion as part of the assimilation process. For the mainstream Protestant denominations, the number of converts was not enough to prevent a decline in membership but, for evangelical Protestantism, it was.

What will be the impact of an effective wall between the USA and Mexico? These trends from the past eight years suggest that the number of Christians in the USA will decline faster than it has been. This will first be evident in Catholicism, but it will soon show up in the membership of the mainstream Protestant denominations and, then, among evangelical Protestant Christians.

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