WASHINGTON (CNS) – A major study of the religious landscape of the U.S. shows a continuing decline in the number of people who consider themselves part of any religion, with the largest shift occurring among the “millennial” generation.

The Pew Research Center survey of 35,000 people, conducted in 2014, found that the percentage of Americans who identify themselves as Christians declined by eight percentage points since the last religious landscape survey in 2007. The first data from the survey, released May 12, dealt primarily with religious affiliation. Future reports will address other parts of the survey, such as religious beliefs and practices.

The phenomena of people changing religions also has become more pronounced, the survey found, and said that is especially true for people who were raised Catholic.

“Nearly one-third of American adults (31.7 percent) say they were raised Catholic,” the report said. “Among that group, fully 41 percent no longer identify with Catholicism. This means that 12.9 percent of American adults are former Catholics, while just two percent of U.S. adults have converted to Catholicism from another religious tradition. No other religious group in the survey has such a lopsided ratio of losses to gains.”

The report said the number of people who define themselves as religiously unaffiliated changed from 16 percent in 2007 to 23 percent in 2014.

Among those, the 51 million Catholics represents a decrease of about three million, or from 24 percent of the population to 21 percent.

The study noted that the figure might be somewhat explained by the statistical margin of error, and could be as little as a decline of one million people.

It also added that Catholics’ percentage share of the population has remained relatively stable over decades, in comparison to Protestants, who have steadily declined.

One critic said Pew’s figures for Catholics don’t reflect what other polls by Gallup, Public Religion Research Institute and the General Social Survey have found.