During a historic day on the U.S. senate floor, Kyrsten Sinema (D-AZ) became the first female senator of Arizona, and the first openly bisexual member of the senate.

Sinema took her ceremonial oath of office holding her hand on a copy of the Constitution rather than a religious text such as the Bible.

Vice President Mike Pence ended Sinema’s oath by saying the standard words, “so help you God?,” to which Sinema responded, “I do.”

A spokesperson for the senator said that the book was from the Library of Congress and contained texts of the U.S. and Arizona constitutions.

“Kyrsten always gets sworn in on a Constitution simply because of her love for the Constitution,” Sinema’s spokesman, John LaBombard said, while not addressing Sinema’s religious views.

By contrast, her colleague U.S. Sen. Martha McSally (R-AZ) took her oath using a Bible recovered from the USS Arizona, which sank during the attack on Pearl Harbor in 1941.

Sinema is the only member of Congress who openly identifies as religiously unaffiliated, according to the Pew Research Center for Religion and Public Life.

But The Arizona Republic points out that Sinema’s move to forgo using a religious book to take her ceremonial oath, as most members of Congress typically do, may only fuel ongoing speculation that she is an atheist.

The 116th Congress is the most diverse in history, with incoming members expanding political representation on a number of fronts.

For the first time, Congress now includes Muslim women, Native American women, an openly bisexual senator and the youngest woman ever elected to serve in either chamber.

Yet despite that progress, Congress is once again beginning a new session without a single member who openly identifies as atheist, according to a Pew Research Center analysis of the latest CQ Roll Call “Faith on the Hill” survey.

Sinema is the only Member of Congress who says she is religiously unaffiliated.

An additional 18 members of the 116th Congress responded “don’t know” or didn’t list a religious faith on the latest questionnaire.

Sinema, who has taken issue with the “atheist” label in the past, has been the only openly unaffiliated member of Congress since she was elected to the U.S. House in 2012.

The survey underscores a longstanding lack of political representation among the growing ranks of Americans who say they don’t believe in God, are questioning the existence of a higher power, or simply don’t subscribe to traditional organized religion.

Nearly one-quarter of U.S. adults now consider themselves to be religiously unaffiliated, compared to just 0.2 percent of Congress, or 1 in 535 members, according to Pew.

Meanwhile, more than 88 percent of Congress identifies as Christian ― including all but two Republicans, both of whom are Jewish ― compared to just 71 percent of the U.S. public.

Overall, the 116th Congress is slightly more religiously diverse than the prior Congress.

This gap in representation has persisted as atheists, nontheists, agnostics and other so-called religious “nones” continue to battle for public acceptance following decades of social and political prejudice toward their views.