Religiosity in the U.S. is in sharp decline, according to a study released by the Pew Research Center on Thursday, with the ranks of people who don’t adhere to any faith growing fast while church attendance has fallen steeply.

Christians make up 65% of the U.S. adult population, according the 2018-2019 study, down from 77% in 2009. At the same time, those who don’t identify with any religion—often known as “nones”—now make up more than a quarter of the population, compared with 17% a decade ago. Only 45% of adults said they...