Church attendance has been declining for a number of years - it's not worth measuring atheism in empty pews.

In US surveys on religious beliefs, respondents increasingly answer with 'no religion', and these 'nones' consitute the largest demographics in these surveys.

In a report theatrically titled Exodus, the Public Religion Research Institute (PRRI) explored the reasons people become atheists, and the diversity of the 'nones' coalition.

'Nones' or 'unaffiliates' are harder to categorise because rather than subscribing to a particular sect, they're defined by the absence of a sect.

Surveys which focus on the percentage of people identifying as 'no religion' usually fail to recognise the diversity within that label - respondents could have abandoned religion or they simply may be theistic but not attend church.

The PRRI records that 25 per cent of Americans claim no formal religion, up from just six per cent in the 1970s and from 20 per cent as recent as 2012.

Although more children are being raised without any religion, the number of 'nones' who were once religious remains high.

Here are six reasons cited by the report as to why they left:

1. Your church congregation became too focused on politics

Sixteen per cent of respondents to the PRRI's survey cites their congregation's political focus as the reason they chose to no longer affliate.

It shows that ardent evangelism and activism is more of a turn off, than a simply explanation that atheists decide to 'hate god'.

2. The clergy sexual abuse scandal

The survey showed that 19 per cent felt that the sexual abuse acts committed by clergy was an important reason for leaving their church. For those raised Catholic, this number was 32 per cent.

Last year, Pope Francis told victims of sexual abuse that "God weeps for the sexual abuse of children", and that:

All responsible will be held accountable.

3. Negative religious teachings about or treatment of gay and lesbian people

Homophobia from some pulpits, and faith motivated acts of homophobia was an important reason for leaving for 29 per cent of respondents.

Those who were raised Catholic are more likely than those raised in any other religion to cite this (39 per cent).

Picture: (NICHOLAS KAMM/AFP/Getty Images)

4. Your family was never that religious growing up

Lack of an evangelic household was a reason for almost a third of respondents (32 per cent).

5. You have a university degree

The study divided people who were 'unaffiliated' into subgroups. These were 'Rejectionists', 'Apatheists' and 'Unattached Believers'. Rejectionists, read atheists, were more likely than any other group to have a degree from university (38 per cent). By contrast, unattached believers, those without a church but who believe in some form of divine power, were the most likely to have a high school degree or lower education (65 per cent).

6. You stopped believing in religious teachings

For a majority of the 'nones' who were raised with religion, the simplest explanation was the most common.

Sixty per cent of people involved in PRRI's survey attributed foregoing their religious identity to losing faith and belief in religious teachings.

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