John Prescott had a barbed response yesterday when he heard that his old boss Tony Blair had at last converted to Catholicism.

“Well, it doesn’t come as any surprise to me,” said the former deputy prime minister. “Good luck to him and his family. Of course, there was a point when Tony was walking around with a Koran and a Bible. It looks like Catholicism has won out in the end.”

Some Catholics were not smiling, however. There have long been objections among some of the faithful to the welcoming into their church of a prime minister who pushed through legislation that was at odds with its teaching.

They want to know whether he has come to the church or whether the church – desperate for such a high-profile convert – has come to him.

On abortion, stem-cell research and civil partnerships for homosexuals, Blair’s record in office was well outside the Catholic pale. Yet barely six months after leaving No 10, he has undergone a solemn rite that involves swearing the acceptance of Catholic doctrine in its entirety.

“My question would be, has he changed his mind?” Ann Widdecombe, the Conservative MP and fellow convert, asked yesterday.

Canon Timothy Russ, a close confidant of Blair, seemed sure that he had had a change of heart. “It’s a great joy – I’m lost for words. This is wonderful, wonderful news – our family has a new member. But I fear there will be a lot of moaning from Catholics, who criticised him for undermining marriage and not respecting unborn life in the womb. He will have admitted to himself that his previous views were not right,” Russ said.

Peter Kilfoyle, the Catholic Labour MP, said that he had other doubts, however.

“I’m trying to think of a parable about this, but I don’t think there is one to match it,” Kilfoyle said.

“Put it this way: if he showed just one ounce of contrition over Iraq, then he would be closer to the body of morality that is the Catholic church.” It is perhaps one of the most high-profile conversions since St Paul on the road to Damascus. But it was also one of the slowest.

Brought up an Anglican, Blair has been attending mass with his wife, Cherie, and children for at least 20 years. Speculation about his conversion has been rampant since he became prime minister 10 years ago.

Vittorio Messori, a Catholic writer who co-authored books with John Paul II and Benedict XVI, said: “Friends in the English church have told me that it all started with Cherie’s faith. Gently, gently, she convinced him to go to mass with her so as not to separate the family on Sundays. He was struck above all by the liturgy. I found out about this 10 years ago.

“I used to think Blair went to please his wife. Then five years ago I was told that when he went abroad, Blair told his staff to find a church for him to go to mass on a Sunday – even when Cherie wasn’t with him.

“It’s certain that Blair decided to wait until he left office to announce his conversion. There are ugly precedents for a ruler changing his religion – look at Henry VIII. I think in his conscience Blair had been a Catholic for some time, but he waited until he no longer had the job of prime minister.

“When he met Benedict in June before leaving office, they spoke alone for at least a quarter of an hour. Blair told the Pope of his intention to officially become a Catholic.

“I’m very pleased for Blair. I appreciate his courage because today you need courage to profess the faith.”

Blair himself has admitted as much. He professed his Christian belief, but always appeared uncomfortable when asked about it in public. Earlier this year he said on television that he had avoided talking about his religious views while in office for fear of being labelled “a nutter”.

Alastair Campbell, Blair’s director of communications, was not comfortable with it either, declaring four years ago “we don’t do God”, although he has since said that his former boss “does do God in a big way”.

Kilfoyle sympathised: “He was scared that people might have thought he was a nutter if he converted while in office. To be honest, I think he was right. This is an agnostic country and doesn’t deal with ‘God-botherers’ very well.”

Catherine Pepinster, editor of The Tablet, the Catholic magazine – which last month accurately forecast the date and place of his conversion service – gave an insight yesterday into why he had not converted while in office.

“I understand one of the issues he was concerned with, because he was so closely involved in negotiations over peace in Northern Ireland, [was] that perhaps some people there might have been uncomfortable with the prime minister converting to Catholicism at such a time,” she said.

“This situation is different. Although he remains a public figure and clearly has a role to play in the Middle East, it isn’t perhaps quite the same.”

One other question that has to be asked is how Blair fitted the formal preparation for conversion into his busy life as peace envoy to the Middle East and as a highly paid star on the international lecture circuit.

As The Sunday Times reported last week, he is making between £500,000 and £1m a month from public speaking engagements, matching the earning power of Bill Clinton, the former US president. Blair’s schedule of up to five speeches a month has been dovetailed into his work shuttling around the Middle East.

One day earlier this month he had meetings with four Israeli ministers, then dinner with Condoleezza Rice, the US secretary of state. On another day he breakfasted in Saudi Arabia, lunched in Kuwait, went to Qatar for a 30-minute television interview and flew on next morning to Israel.

How has he managed to fit in the soul-searching required for his conversion?

John Humphrys, the broadcaster, sees no problem. “Tony has been a Catholic in his private thoughts and private life for such a long time,” he said.

Some Catholics think it matters a great deal that Blair has thought about his actions in office and has repented.

John Smeaton, director of the Society for the Protection of Unborn Children, told The Spectator magazine last month: “We need to hear a full repudiation from him. Without one, having Blair as a Catholic is like having a vegetarian in a meat-eating club. It simply does not make sense.”

Usually the conversion process is a programme called the RCIA – rite of Christian initiation for adults. It lasts at least a year and involves going to meetings and receiving instruction. But, according to John Allen, senior correspondent for the National Catholic Reporter, Blair followed another path.

“Blair didn’t go through the RCIA,” said Allen. “In any case, it is not required by canon law. All canon law demands is that you present yourself, that you make the request with a free will and then there’s a ritual which any priest can perform. In Blair’s case, like in any other case, all it takes is one of the priests who has been following him saying that he is ready to be received.”

Blair began his spiritual preparation for conversion in July, a month after he stepped down. He was instructed by Monsignor Mark O’Toole, private secretary to Cardinal Cormac Murphy-O’Connor, head of the Catholic church in England and Wales.

Over the next five months the two men discussed Catholicism at the Blairs’ Connaught Square home in London on Fridays or at weekends when the former prime minister was in the UK. They held 25 half-hour conversations that followed a formal programme: the trinity, the incarnation, morals and the foundation of the church. There were separate conversations about personal issues.

Early on Friday evening, everything was at last in place. Blair stood next to his wife, who acted as his sponsor, in the ornate chapel of the Archbishop’s House, Westminster.

His family and friends were there, as was Monsignor John Walsh, a chaplain at Cranwell RAF college who occasionally took communion for the Blairs at Chequers. O’Toole and the cardinal officiated.

The high point of the ceremony is a declaration of faith: “I believe and profess all that the holy Catholic church believes, teaches and proclaims to be revealed by God.”

This is what sticks in the craw of traditionalists such as Widdecombe. But Monsignor Andrew Faley, assistant general secretary to the Catholic Bishops Conference of England and Wales, said the profession of faith “does not mean we have to go over his past actions and ask him to explain them all again. The future starts here”.