More people lie about attending church which is the only way you can get the 40% figgure. But actual attendance has not been keeping up with population growth.
"Q: How many people go to church each Sunday?
A: For years, the Gallup Research Organization has come up with a consistent figure — 40 percent of all Americans, or roughly 118 million people, who said they attended worship on the previous weekend. Recently, sociologists of religion have questioned that figure, saying Americans tend to exaggerate how often they attend. By actually counting the number of people who showed up at representative sample of churches, two researchers, Kirk Hadaway and Penny Marler found that only 20.4 percent of the population, or half the Gallup figure, attended church each weekend.
As added proof for the accuracy of this smaller percentage of churchgoers, if 20.4% of Americans (approximately 63 million in 2010) attended the nation's 350,000 congregations weekly then the average attendance would be 180 people per congregation which is almost exactly the figure that numerous research studies have found."
This is a study of dishonesty more then attendance. You can't go and just ask, "Do you go to church every weekend. People will lie. Also read the table of church growth over the past 40 years on the page you just quoted. Christianity falling is just as much a "Known Truth" as much as above 50% divorce rate of first time marriages.
Care with studies. Most Christians "just know" Christianity is falling away and pay big money to people who predict the death of the faith (George Barna AKA father of Christians divorce rate is the same or higher then non-Christian divorce rate though the one issue is his numbers were all a lie). The truth is that it has grown in mind blowing numbers and really hasn't slowed down except in Mainline denominations and Catholic Churches. The vast majority of stats are if people were honest to the question did they go to church. The surprise is most people are dishonest that they attend church weekly. Case in point East Sunday attendance is huge compared to normal weekly attendance.
I think people are just being blind to the meteoric rise of Christianity in the past 100 years. In 1910 more then half the Christians in the world were European.
"Q: How many people go to church each Sunday? A: For years, the Gallup Research Organization has come up with a consistent figure — 40 percent of all Americans, or roughly 118 million people, who said they attended worship on the previous weekend. Recently, sociologists of religion have questioned that figure, saying Americans tend to exaggerate how often they attend. By actually counting the number of people who showed up at representative sample of churches, two researchers, Kirk Hadaway and Penny Marler found that only 20.4 percent of the population, or half the Gallup figure, attended church each weekend. As added proof for the accuracy of this smaller percentage of churchgoers, if 20.4% of Americans (approximately 63 million in 2010) attended the nation's 350,000 congregations weekly then the average attendance would be 180 people per congregation which is almost exactly the figure that numerous research studies have found."