There has been widespread speculation that Mr Blair, an Anglican, is planning to convert to Catholicism once he leaves office.
His wife, Cherie, and four children are all Catholics, and he is known to attend Mass with them at Chequers.
Iraq and the Middle East were to be on the agenda for the talks.
Mr Blair has reportedly talked about the possibility of conversion with a priest, and has participated in a private service at the Vatican during a previous trip to Rome.
But in a newspaper interview, Mr Blair insisted the issue of his faith is 'unresolved'.
Asked if he would be converting, Mr Blair replied: 'I don't want to talk about it. It's difficult with some of these things. Things aren't always as resolved as they might be.'
He also said he was 'nervous' discussing the issue before today's meeting.
There is no constitutional barrier to such a conversion although it would have made Mr Blair Britain's first Catholic Prime Minister.
It is suggested that he would wait until leaving Downing Street to avoid potential clashes such as over his role in the appointment of Church of England bishops.
Mr Blair's audience with Benedict XVI at the Vatican will be the final foreign engagement of his 'farewell tour', with Gordon Brown due to get the keys to Number 10 on Wednesday.
Downing Street has said Mr Blair is keen to discuss 'interfaith issues' at the meeting.
