An off-beat Christian website has been sending 80 "mystery worshippers" to review churches across London.
The congregations are unaware that a reviewer is among their number
Reporters acting as worshippers mingle with unsuspecting congregations for website Ship of Fools.
Site editor Simon Jenkins told BBC Five Live the aim was to give non-churchgoers an insight into what goes on at Sunday services.
He said each report combined physical descriptions of the church with the spiritual moments of the service.
Mr Jenkins said the mystery worshippers take a 20-point questionnaire to each church service, which includes questions such as "How hard was the pew?", "How hot was the coffee afterwards?" and "How warm was the welcome?".
The questionnaire also asks "What part of the service felt like heaven?" and "What part of the service felt like the other place?".
Reporters also take in a card with a picture of a lone-ranger type of figure on it, and the words "you've been blessed by a visit from the mystery worshipper, read about your church soon on shipoffools.com".
The card is placed in the collection plate or bag during collection time.
Mr Jenkins said the project, which has been running for about seven years, had been met with mostly positive comments from churches.
"I think churches recognise that in the modern world they have to sharpen up a bit in terms of their communication skills and especially in the way they welcome visitors to church," he said.
According to Mr Jenkins, some ministers have read out their mystery worshipper report from the pulpit or made it the basis for a sermon, while others have used it to look at how they can improve.
He said the idea behind the reviews is to give people who do not attend church an insight into what goes on, and also to give others an idea of which church might suit them best.
Mr Jenkins said the reports mix amusing, quirky stories from services with the spiritually sublime.
"There's plenty of space in the report actually for people to record the really spine-tingling moments of worship which do happen.
"We had a report recently from a church in Poland and the reporter said the silence was palpable, and it was an incredible feeling that he'd never had in church before, you know a kind of genuine meeting with God.
"The kind of tone of mystery worshipper is a bit mischievous, and often our reporters report the most funny things that happen in church .
"But also there's an opportunity to record genuine spiritual moments and that's fabulous as well."