From Michael Barone:
In the 2004 presidential exit poll, 74 percent of voters described themselves as churchgoers, 23 percent as said they were evangelical or born-again Protestants and 10 percent said they had no religion.
This is in line with longer trends. Roger Finke and Rodney Stark in “The Churching of America 1776-1990” used careful quantitative analysis to show that in America’s free marketplace of ideas, the religions and sects that have grown are those that make serious demands on members. Those that accommodate to secular critics and make few demands decline in numbers. The Roman Catholic Church continues to grow in America; the Assemblies of God and the Mormon Church grow even faster. But mainline Protestant denominations, which spend much effort ordaining gay bishops or urging disinvestment in Israel, lose members
Who inherits the future? In free societies, each generation makes its own religious choices, but people tend to follow the faith of their parents. Secular Europe, with below-replacement birthrates among non-Muslims, could be headed for a Muslim future, as historian Niall Ferguson suggests.
In the United States, as pointed out by Phillip Longman in “The Empty Cradle” and Ben Wattenberg in “Fewer,” birth rates are above replacement level largely because of immigrants. But, as Longman notes, religious people have more children than seculars. Those who believe in “family values” are more likely to have families.
In other words, if your parents had fewer children, chances are you will too.