Blair was scared of being branded a religious nutter.
He reveals the truth about his convictions tonight in a BBC1 documentary entitled The Blair Years.
While in the US and some other countries, it is considered natural for politicians to be open about their faith, he explains.
"But you talk about it in our system and, frankly, people do think you're a nutter," says Mr Blair.
"Voters tend to think religious politicians go off and sit in the corner, commune with the man upstairs, and then say: 'Right, I've been told the answer and that's it.'"
He says that his faith was 'hugely important' in dealing with big issues, and refutes spin doctor Alastair Campbell's famous remark that: "We don't do God".
"It was hugely important," he says. "There is no point in me denying it, I happen to have religious conviction.
"I don't think there is anything wrong with that. I think it is a strength for people."