According to the study, which is an analysis of nearly 43,000 interviews conducted in recent years by the Ventura, Calif.-based organisation, more than 70 per cent of American adults describe themselves as Christians.

Only 63 per cent of people rank "low" on the Barna Group's post-Christian scale, however, while 28 percent are considered "moderately" post-Christian and nine percent are considered "highly" post-Christian.

The post-Christianity scale is based on 15 faith-related metrics researchers have tracked in recent years. Included in these metrics are measurements indicating the percentage of people who have not prayed to God in the last year (18 per cent), who haven't read the Bible in the last week (57 per cent), who don't consider faith an important part of their lives (13 per cent) and haven't been to a Christian church in the last year (33 per cent), among other things.

Individuals whose beliefs and behaviours matched nine or more of the 15 characteristics were labeled "post-Christian", while those who met 12 or more were labelled "highly post-Christian". David Kinnaman, president of Barna Group, explained the purpose of trying to measure the level of post-Christianity in a statement on the group's website.

"First, we wanted to expand the scope of secularisation beyond what people call themselves," said Kinnaman. "Faith-oriented self-descriptions are fine, but they are really only skin-deep in terms of understanding faith. In addition to identity, we also wanted to account for two other critical aspects of faith: belief as well as behaviour.