Moral crusaders must maintain impeccable standards or risk being outed as sanctimonious phoneys – and nothing damages a cause like hypocrisy. This is neither canon nor criminal law, but the law of common sense.
When Cardinal Sean Brady sets himself up as a moral authority, lecturing TDs on their duties as lawmakers and Catholics on their obligation to oppose abortion legislation, naturally his credentials are relevant.
In theory, his role as the Catholic Church's Primate of All Ireland should mean his voice is an influential one in the abortion debate.
But Cardinal "I was only following orders" Brady has feet of clay: a handicap so conspicuous that it renders him unfit to sermonise on ethics.
This man is compromised irreversibly by his role in the hierarchy's culture of silence and cover-up over clerical child abuse. He lacks credibility, which means his views – even where sincere and valid – have a currency shortfall.
He ought to have resigned in 2010 when his role in the secret church investigation into notorious paedophile priest Brendan Smyth was revealed. Retirement to a monastery would have been fitting – and if he didn't go willingly, the Catholic Church ought to have despatched him.
Its blunder in retaining the cardinal smacks of chickens coming home to roost, because during this groundbreaking social discussion his voice is neutralised at best – and a provocative intervention at worst.
It is manifestly difficult to listen to his use of terms including "self-deception" and "convenient compromise", in relation to abortion legislation, without cringing at his blatant dual standards. Both terms, bandied about by him at a service in Knock last weekend, apply equally to his attitude to the Catholic Church's handling of paedophile clergy.
In case the details of his involvement in the Brendan Smyth case are forgotten already, let me recap. Cardinal "I was only following orders" Brady was part of a 1975 canon law inquiry into Smyth – one so bent on protecting Mother Church's interests that the predator was allowed to proceed unchecked for many years.
Then Father Brady, he was a note-taker, and argues he had no authority over Smyth. However, a BBC2 documentary 'The Shame of the Catholic Church', last year reported that he was instructed to check out a 14-year-old boy's story about being abused by Smyth.
That boy also gave the inquiry the name of a 15-year-old. Brady interviewed that victim in a parochial house – without any attempt to inform the parents or ask for their permission. Afterwards, the 15-year-old was sworn to silence – as the 14-year-old had been. Their sexual abuse was compounded by an abuse of power.
Here's an example of churchmen dancing on the head of a pin, and explains why churches are emptying out in Ireland. The Catholic Church insists the boys were told only to keep quiet about the inquiry – not about the abuse. As though an intimidated, abused child would know the difference.
Yet, presumptuous to the end, Cardinal "I was only following orders" Brady continues to moralise on ethics.
At Knock, he said: "In this Mass we pray for courage – the kind of courage that is needed to look the truth in the eye and to call it as it is, without yielding to self-deception or bowing to convenient compromise, scrupulously avoiding ambiguous language which cloaks the true horror of the situation and reduces its seriousness in public."
Imagine how it would be viewed if a member of one of Bertie Ahern's cabinets was in government today, imposing austerity. Consider how people would react if the top tier of bankers hadn't been cleared out. The cardinal is damaged goods.
Yet he seeks to influence TDs on the government benches, telling legislators they have an obligation to oppose laws which undermine the right to life.
This, from a man who played a role – a minor one, he cries – in shielding a flagrant paedophile whose collar gave him access to children. Even if we accept the explanation that the serial offence nature of paedophilia was unknown at that time, the true horror is understood today. In hindsight, the cardinal must be aware of the gravity of the smokescreen he participated in erecting.
The Catholic Church is not awash with paedophiles. There are many hardworking and honourable clergy. But the hierarchy's entrenched attitude, even today, is hard to stomach. Its failure to remove Cardinal "I was only following orders" Brady was – and remains – profoundly shocking.
The Catholic Church has no hope of regaining ground while it allows itself to be represented by such men, the voice of Archbishop Diarmuid Martin, as Primate, would be less easy to ignore.
As for loose talk on the part of the Catholic Church about suing the State over abortion legislation, or this hare that's been raised about excommunicating politicians – these add up to yet another own howler. What next, threatening Enda with the Inquisition?
The Catholic Church is entitled to give its views. But no religion has the right to seek to control a state, or impose its morality on everyone living there.
Urging politicians to set aside the will of the people, as expressed by referendum, is wrong.
It mystifies me why ordinary, decent Catholics – some of whom privately admit they are unhappy with the cardinal's continuation as primate – don't distance themselves from his hypocrisy. They could boycott his Masses, refuse to take communion from him, protest with placards outside his place of residence, or send petitions to the Vatican. If they are silent, where is the incentive for the Catholic Church to put its house in order?
As for Cardinal "I was only following orders" Brady, he admits what he did – he just doesn't see anything wrong with it. And there, in a nutshell, is where his integrity withers.