Three out of four Americans call themselves Christian, according to the American Religious Identification Survey from Trinity College in Hartford, Connecticut. In 1990, the figure was closer to nine out of 10 -- 86 percent.
At the same time there has been an increase in the number of people expressing no religious affiliation. The survey also found that "born-again" or "evangelical" Christianity is on the rise, while the percentage who belong to "mainline" congregations such as the Episcopal or Lutheran churches has fallen.
One in three Americans consider themselves evangelical, and the number of people associated with mega-churches has skyrocketed from less than 200,000 in 1990 to more than 8 million in the latest survey. The rise in evangelical Christianity is contributing to the rejection of religion altogether by some Americans, said Mark Silk of Trinity College.
"In the 1990s, it really sunk in on the American public generally that there was a long-lasting 'religious right' connected to a political party, and that turned a lot of people the other way," he said of the link between the Republican Party and groups such as the Moral Majority and Focus on the Family. "In an earlier time, people who would have been content to say, 'Well, I'm some kind of a Protestant,' now say 'Hell no, I won't go,'" he told CNN.
Silk also said the revelation that some Catholic priests had sexually abused children -- and senior figures in the church hierarchy had helped to hide it -- had driven some Catholics away from religion. And, he said, it is now more socially acceptable than it once was to admit having no religion.
"You're not declaring yourself a total pariah. The culture has changed in a way that makes it easier to say, 'No, I don't have a religion. Even in the past year, Mitt Romney and Barack Obama feel obliged to talk about 'those of no faith,'" he pointed out.
Obama mentioned people without faith in his inaugural address in January, making him the first president to do so.
In the survey, one in five Americans said they have no religious identity or did not answer the question, and more than one in four said they do not expect to have a religious funeral.
The rise in what the survey authors call "nones" is the only trend reflected in every single state in the study, Silk said. "We don't see anything else in the survey that is nationwide," he told CNN.
Other findings include:
• The percentage of Catholics in the United States has remained steady at about one in four since 1990, while the percentage of other Christians has plummeted from 60 percent to 50 percent.
• The percentage of Muslims has doubled since 1990, but remains statistically very small, only 0.3 percent in the original survey and 0.6 percent today.
• Mormons have remained steady as a percentage of the population, even as the number of people in the United States has grown. They make up 1.4 percent of the population.
• The number of Jews in the United States is falling if the category includes only those who define themselves as Jews religiously, but has remained the same if the category includes people who consider themselves ethnically Jewish.
The survey polled 54,461 Americans between February and November of last year.
Pollsters conducted the research in both English and Spanish. The survey is the third in a series, following polls in 1990 and 2001.