In his final days as Prime Minister, Tony Blair went to Rome and visited the Pope. As a gift he gave Pope Benedict a signed photograph of the 19th-century Roman Catholic convert John Henry Newman. Newman had been one of the leaders of the Oxford Movement within the Church of England before his reception into the Catholic Church in 1845. In 1879 Pope Leo XIII made him a cardinal. Many have been influenced by him and the Vatican may soon declare him “Blessed”, a decisive step on the path to sainthood.

Some saw Tony Blair’s gift as a clear sign that he would soon be received as a Catholic himself. And now that he has been, the news has sparked critical comments, especially concerning the way he made allowance for policies as Prime Minister that clashed with Catholic teaching, for example, on abortion, on stem-cell research, and on same-sex unions. His sincerity has been questioned. How could he have supported those policies then, yet become a Catholic so soon after?

One television commentator after mentioning those very issues, then asked whether, had Blair been a Catholic in 2003, he could have gone to war in Iraq. He referred to Pope John Paul II’s known opposition to the war. The implication was clear. Although Prime Minister, as a Catholic, would Blair not have been bound to obey the Pope? It is not a new question. It was raised by William Gladstone in 1874.

Gladstone had lost a general election and he attributed his misfortune in part to the influence that he felt the Pope had exercised in Ireland, an influence, he believed, that had undermined his position. He was mistaken. But smarting from his defeat he produced a popular pamphlet attacking the decrees of the first Vatican Council (1869-70), which had defined Papal Infallibility, and claiming that on account of those decrees Roman Catholics had forfeited their moral and mental freedom and placed their civil loyalty and duty under the Pope. As citizens they were no longer to be trusted.

Faced with this attack, many Catholics asked Newman to reply. It was difficult for him. He knew and admired Gladstone. Although he believed him mistaken in this instance, he did not wish to give personal offence. (Gladstone later assured him that he had not.) Nevertheless, he eventually felt compelled to write, and did so in his public Letter to the Duke of Norfolk.

At the same time Newman also wanted to offer an alternative view to some of the exaggerated claims for papal authority then prevalent among Catholic extremists. He saw Gladstone’s outburst as a chance to set the record straight. By showing the extent of papal claims, he wanted also to show their limits.

One of the questions he discussed was that of divided allegiance. Could Catholics be loyal or were they always untrustworthy? First, there was the general matter of the Pope exercising “the supreme direction” of Catholics. Newman pointed out immediately that “supreme” is not “minute”. Papal authority may be supreme, guiding what Catholics are to believe, their faith, and how they are to behave, their morals, but without intruding minutely into the details of their daily lives. By way of contrast, he observed, consider civil law. That is far more intrusive. “There are”, he pointed out, “numberless laws about property, landed and personal, titles, tenures, trusts, wills, covenants, contracts, partnerships, money transactions, life insurances, taxes, trade, navigation, education, sanitary measures, trespasses, nuisances, all in addition to the criminal law.” Yet these laws, he went on, are not regarded as interfering “either with our comfort or our conscience”. He was writing in 1875. Are we as much at ease with our laws today? But certainly the Pope is not interfering with our ordinary daily life.

Then there was a second question, touching the imagined clash between papal teaching and parliamentary legislation. Newman struggled to find an example. We may have less difficulty and refer to the very issues for which Tony Blair has been criticised, abortion, stem-cell research and same-sex unions. But Catholic politicians in raising their concerns about these controversial matters are not simply bowing to Rome in blind obedience. A debate needs to continue. It is not obvious that the tragedy of an unwanted pregnancy is best resolved by the further tragedy of termination; that the ethical issues surrounding the use of stem cells are empty of meaning; and that the way legislation is framed for same-sex unions can do no harm to marriage and family life. Catholic politicians should never seek to impose their views on others, but in a free and democratic society they have the right to argue their case and hope to be heard in an honest, open-minded way.

And then there is that third specific issue. Would Tony Blair as a Catholic have been unable to go to war in Iraq given Pope John Paul’s opposition to it? Would his policy have been controlled by the Vatican? Newman raised the question directly. His answer was plain. He refers to members of the Armed Forces, but the point applies to prime ministers as well: “ . . . were I actually a soldier or a sailor in Her Majesty’s service, and sent to take part in a war which I could not in my conscience see to be unjust, and should the Pope suddenly bid all Catholic soldiers and sailors to retire from the service, here again, taking the advice of others, as best I could, I should not obey him.”

In fact Newman regarded the notion of a genuine conflict between obedience to Parliament or obedience to the Pope as unreal. He recognised that some exceptional situation could occur, but were it ever to do so, he argued, the individual case would need to be judged on its merits. The irony of the case of the war in Iraq is that many people wish that Tony Blair had been influenced by the Pope rather more.

Monsignor Roderick Strange is the Rector of the Pontifical Beda College, Rome. His book, John Henry Newman: A Mind Alive , will be published by Darton, Longman and Todd on January 21.