The proportion of Americans who say they are "absolutely certain" God exists has dropped sharply from 71 percent to less than two thirds, the Pew Research Center said Tuesday.
The share of US adults who say they believe in God declined from 92 to 89 percent over the same period, from 2007 to 2014, but is still remarkably high compared to other developed countries.
The percentage of Americans who say they pray every day, attend regular services and consider religion very important have also clocked small, but significant declines, the research center said.
The vast majority of Americans, 77 percent of adults, continue to identify with some religious faith, but a growing number are unaffiliated to a particular group, the study found.
Younger Americans are also less religious than their elders. For example, four in 10 of the youngest Millennials say they pray every day, compared to six in 10 Baby Boomers.
The survey also showed that nearly all major religious groups have become significantly more accepting of homosexuality in recent years -- even evangelicals and Mormons who traditionally have expressed strong opposition to same-sex relationships.
Changing attitudes about homosexuality are linked to wider acceptance among the younger generation than older adults.
The study also showed that the religiously unaffiliated -- the majority of whom still believe in God -- are now more numerous among Democrats and Democratic-leaning adults than Catholics, evangelicals and Protestants.
The religiously unaffiliated are growing less quickly within the Republican party, where they are outnumbered by evangelicals, Catholics and mainstream Protestants, Pew said.
The poll was carried out among a representative group of 35,071 adults interviewed by telephone from June to September 2014 and has a margin of error of 0.6 percentage points, Pew said.