The number of Americans who identify as having no religion has risen 266 percent since 1991, to now tie statistically with the number of Catholics and Evangelicals, according to a new survey.

People with no religion – known as 'nones' among statisticians – account for 23.1 percent of the U.S. population, while Catholics make up 23 percent and Evangelicals account for 22.5 percent, according to the General Social Survey.

Those three groups now represent the largest the religious groups in America.

The survey has tracked a broad swath of American trends since 1972, offering comprehensive insight into the evolving face of religion over more than four decades.

Ryan Burge, a political science professor at Eastern Illinois University who analyzed the data, said that experts have several theories about why the number of 'nones' has risen so dramatically in recent decades.

'One of them is that many people used to lie about what they were,' he told DailyMail.com. 'Many people were (always) atheist or non-religious, but it was previously culturally unacceptable to not have a religion in America.'

This graph illustrates the shift in religious ideologies since 1972, with a sharp decline in the number of mainline Protestant Christians and a dramatic uptick in the number of people with no religion. Known as 'nones' those religion-free Americans are now statistically tied with Catholics and Evangelicals as the largest religious groups in America

Shifting political ideologies about social issues has also played a role, with fewer Americans comfortable with the rhetoric of their religious leaders.

'Another (theory) is that the religious right kind of cleaved moderate Christianity and a lot of moderate Christians who were moderately attached said they didn't want to defend Jerry Falwell … and all the anti-gay and anti- abortion religious rights leaders,' Burge said. 'So they said, 'You know what? I'm out.'

As the 'nones' have ascended, the number of mainline Protestant Christians has fallen 62.5 percent since 1982, to now account for just 10.8 percent of the U.S. population, according to the survey.

The number of Catholics has gone up and down over the decades, cumulatively decreasing more than 4 percentage points from 27.3 percent in 1972.

However, that indicates an overall stability for that religion, despite the decades of sexual abuse scandals that have plagued the Church, Burge said.

'Catholicism is more cultural than religious in a lot of ways,' he said. 'People are less and less likely to disaffiliate from Catholicism than Protestantism, which is less cultural in that people are willing to walk away from it.'

Mainline Christianity is dying. -Ryan Burge, Eastern Illinois University

Burge said that America is on a trajectory to become increasingly less religious going forward – following a pattern that has already emerged in Europe's most developed countries, including those in Scandinavia.

'The big questions is what next in terms of what religion is going to look like in America,' he said. 'Secularization theory argues that as countries become more industrialized and prosperous then the throwing off of religion becomes more normalized.'

Burge has seen the overall shifts first hand in his role as a pastor at an American Baptist church.

'My church is on the decline,' he said. 'We had 50 (congregants) in 2005 and now we have 15. We're probably going to have to close (in a few years).'

'Mainline Christianity is dying,' he added. 'It's at least going away. It makes me feel more comfortable that it's not my fault or my church's fault. It's part of a bigger trend that's happening.'