On a denomination-by-denomination level, the picture gets more complex. While the overall number of strongly affiliated people has stayed stable, that’s because Evangelical Protestants have become more tied to their churches, while Catholics identify less strongly with their faith.

In the 1970s, there was only about a 5-percentage-point difference in how strongly Catholics and Evangelicals felt about their religion, Schwadel said. Today, it’s around 20 percentage points. About 56 percent of Evangelicals currently say they’re strongly affiliated with their religion, while only 35 percent of Catholics say the same.

A few things could be driving these trends, Schwadel said, though the survey did not ask people for their reasons behind their religious devotion. Priest sex abuse scandals in the Catholic Church could have shaken people’s trust in the institution, so that they still call themselves Catholic but distance themselves from the Church. Likewise, the demographics of Catholicism are changing, Schwadel said. There are more Latino Catholics in America today than in the past, and Latinos may be less likely to strongly identify with the institution of the Church than white Catholics.

On the Protestant side, Evangelicals became more visible during the 1990s, as well as more politicized, Schwadel said.