The 115th Congress is back in session, and at least one thing looks the same as usual: 91 percent of its members identify as Christians. This proportion has basically remained constant for more than five decades, as long as this kind of data has been available, according to a new study from Pew Research Center. What has changed is the U.S. population: Only 71 percent of American adults identify as Christians.

Some religious minorities, including Muslims, Hindus, and Buddhists, are slightly underrepresented in Congress relative to their population sizes in the United States, while others, including Jews and Mormons, are slightly overrepresented. But these groups aren’t the source of the demographic mismatch between Congress and the rest of the country. The Americans who are vastly underrepresented in Congress are those who don’t identify with any religion at all: Only one member of Congress, Kyrsten Sinema of Arizona, says she is religiously unaffiliated, while 10 others declined to state their affiliation in surveys and interviews with CQ Roll Call.

There are at least two good explanations for this phenomenon. The first is that religiously unaffiliated Americans don’t vote. Another way of putting that is that young people don’t vote: According to the Public Religion Research Institute, 18-to-29-year-olds are three times as likely to be religiously unaffiliated compared to people over 65. Even though the religiously unaffiliated share of the American population has significantly increased from 14 to 22 percent between 2004 and 2014, the share of religiously unaffiliated voters only increased from 9 to 12 percent. This chart offers a stark illustration of how politically disengaged these voters really are: