Denham “protected his position by threatening to reveal Brennan and Helferty as homosexual lovers”, writes James Miller in his book, The Priests, in which he alleges he was sexually abused by Brennan in 1978 when he was 15.

Mr Miller has called for Catholic schools funding to be stopped until the celibacy rule for priests is removed, saying the celibacy vow poses a blackmail risk because of the church’s structures relating to schools.

Studies suggesting up to 50 per cent of Catholic priests break their vows because of sexual relationships with men or women, or sexual abuse of children, mean “every second priest is living a life of deceit, with the other probably aware”, Mr Miller said.

“This systemic dishonesty makes the church unfit to care for children. With so many clergy compromised where is the moral authority to act when confronted with evidence of abuse within a Catholic school?

“How can a compromised bishop be trusted to report offenders to authorities if the offender can destroy the bishop’s career by publishing details of a secret and longstanding affair with a mistress?”

In his book, released on Monday, Mr Miller writes that he was sexually abused by Brennan while a year 10 student at St Pius X and after transferring from Belmont High School.

He alleges Brennan told him: “There is nothing wrong with what we are doing. But some people just won’t understand. It’d be bad for you.”

“It was as if I was his young boyfriend,” Mr Miller writes.

In the book Mr Miller writes that the abuse had a profoundly negative impact on his life. Although he went on to become a Newcastle University law academic, commercial barrister and author, Mr Miller writes about drinking, self harm, suicide attempts, destruction of relationships and homelessness he links directly to abuse.

“I became more or less constantly angry on the inside. Something was definitely broken,” he writes.

In The Priests Miller writes that nearly 40 years after the abuse he discovered Brennan and Helferty were “lifetime lovers”.

This week he stood by the controversial allegation, saying he relied on evidence contained in his civil action against the church which he declined to make public because it has not yet been put to Maitland-Newcastle diocese.

Rumours of the men’s sexual relationship were raised with the Newcastle Herald by others a number of years ago. Witnesses at Helferty’s funeral described Brennan as a sobbing and devastated man grieving for a dead partner, and not a calm Catholic priest celebrating a soul’s elevation to heaven.

While Brennan’s sister-in-law Patricia Brennan said on Wednesday that she had “never heard such rubbish in all my life” and she was “very proud of the fact” she was the late priest’s sister-in-law, former St Pius X teacher Bill Izzard said “I’d give that quite a bit of credence”.

“I wouldn’t say no to that one. I would form the view the bloke’s telling the truth,” Izzard said.

A Maitland-Newcastle diocese spokesperson declined to respond to questions about the allegation that Brennan and Helferty had a sexual relationship. The spokesperson said the diocese would not make public details about victims of child abuse but "respects their right to disclose aspects of their personal narrative as they choose".

In The Priests Miller argues Brennan did nothing because Denham knew of his relationship with Helferty, when it was still dangerously illegal to be an active homosexual in Australia, and when the Catholic Church not only damned homosexuality as an abomination, but demanded the complete suppression of sexual impulses from its priests. The celibacy rule also leaves priests and bishops in clandestine relationships with women at risk of blackmail within the church, Miller said.

“Denham had the senior leadership of St Pius X right where he wanted them,” he writes.

“Denham protected his position by threatening to reveal Brennan and Helferty as homosexual lovers. I would go further and say that whether he ever actually put the specific threat counts for naught. It would have been enough that Brennan and Helferty believed Denham would do so.”

Denham was approached for comment but did not respond to written questions.  

Solicitor representing many victims of church abuse, Dr Judy Courtin, said celibacy was a child protection risk.

“A spiteful nature combined with some serious dirt on a fellow clergyman, can make a powerful weapon with which to demand silence all round. Such silence can only enable the commission of child sex crimes,” Dr Courtin said.

University of Sydney child protection specialist and law Professor Patrick Parkinson said celibacy was a child protection risk because some men who abused children did so because their vows prevented them from having relationships with adults, so they became “situational” abusers.

“If you are a religious Brother, you’re lonely and your context is teaching in a boys’ school, that’s where you go find it,” Professor Parkinson said.

He said all the evidence was that celibacy was “a hard road to follow”. There were credible reports of 50 per cent of bishops and priests breaking their vows which potentially compromised them within the church.

He rejected the argument that Catholic schools funding should cease until the celibacy vow is removed, saying the school context had changed significantly in the past 40 years, particularly priests’ interaction with children.

“Funding issues should be made on legitimate criteria and the celibacy of priests is not legitimate criteria,” Professor Parkinson said. 

Australian Catholic University Professor Neil Ormerod said breaching celibacy could compromise priests and could pose a child protection risk, but there were more significant reasons why the church should reconsider the celibacy vow based on the mental wellbeing of priests.

“There are many reasons why the church might reconsider the whole issue of celibacy, and the ones around abuse are not the only ones. Many priests never mature as psycho-sexual beings and that is a cause of many difficulties,” Professor Ormerod said.

He rejected the argument for stopping Catholic schools funding because of the child protection risk posed by celibacy, saying it might have been a valid argument 30 years ago but was not today.

In public and reported comments Pope Francis has said he favoured retaining the celibacy rule although “it is a matter of discipline, not of faith. It can change”.

The issue of married priests was in his “diary”, the Pope is reported saying.