Tony Blair avoided talking about his religious views while in office for fear of being labelled "a nutter", the former prime minister has revealed.
Mr Blair said politicians who talk about religion "get into trouble"
In an interview for BBC One's The Blair Years, he said that his faith had been "hugely important" to his premiership.
His ex-spokesman Alastair Campbell once told reporters: "We don't do God."
Mr Campbell has now acknowledged to the programme that his former boss "does do God in quite a big way", but that both men feared the public would be wary.
During the interview, Mr Blair said having faith was a crucial component for him in having the character to take on the prime minister's job.
"For me having faith was an important part of being able to do that," he said.
But while it was commonplace in the US and elsewhere for politicians to talk about their religious convictions, he added, "you talk about it in our system and, frankly, people do think you're a nutter".
British voters imagined that leaders who were informed by religion would "commune with the man upstairs and then come back and say 'Right, I've been told the answer and that's it'".
Mr Campbell's refusal to discuss his faith was not due to any opposition to his beliefs, but because "you always get into trouble talking about it", Mr Blair continued.
Mr Campbell added that the former prime minister always asked his aides to find him a church to attend, wherever he happened to be, each Sunday.
"Because he's pretty irreverent, he swears a fair bit, if he sees a very attractive woman his eye will wander and all that stuff, he doesn't look like your classic religious sort of guy," said Mr Campbell.
But he added: "I think his close circle always understood that there was a part of him that was really, really important.
"On that kind of spiritual level it did inform a lot of what he talked about, what he read... what he felt was important."
Mr Campbell said the UK electorate were "a bit wary of politicians who go on about God".
He had also been concerned that the Conservatives would accuse Labour of trying to claim faith as its own.
Peter Mandelson, a close confidant of Mr Blair, said: "He's not an exhibitionist when it comes to religion but deep inside him it is very, very important.
"This is a man who takes a Bible with him wherever he goes and last thing at night he will read from the Bible."
Sir Menzies Campbell, the former Liberal Democrat leader, suggested that Mr Blair may not have been so politically successful had the relationship between his beliefs and his actions in office been better known.
"The public might have been less willing to give him the triumph of three consecutive general election victories if they'd known the extent to which ethical values would overshadow pragmatism," Sir Menzies said.
The Blair Years will be screened on BBC One at 2215 GMT on 25 November. The episode in which Mr Blair discusses his faith will be broadcast on Sunday 2 December.