Leading article: A few words that could prove a revolutionary moment

According to the New Testament: "Joy shall be in heaven over one sinner that repenteth, more than over ninety and nine just persons." If that is true, there will be much cheer above over Pope Benedict XVI's latest words on the subject of contraception.

Interviewed in a new book, the Pope states that condoms can be acceptable in some circumstances, modifying decades of Catholic dogma on the subject. The example he cites is a male prostitute who is at risk of transmitting HIV. For that group, the use of a condom can be "a first step towards moralisation".

It is a begrudging concession. The Pope maintains that condoms are "not really the way to deal with the evil of HIV infection". He still seems to believe that the root cause of this scourge is sexual immorality rather than a randomly mutated virus for which, tragically, there is still no cure.

And the Pope's emphasis on male prostitutes echoes the Vatican's long refusal to accept that Aids is a crisis that affects both the homosexual and heterosexual communities. As UNAIDS, the United Nations programme on HIV/Aids, pointed out over the weekend, only 4 to 10 per cent of HIV infections result from sex between men. But by accepting that, in some circumstances, condom use can be morally right and an "assumption of responsibility" the Pope has shifted the Vatican's line in a profound way.

If it can be a morally correct choice for a male prostitute to use a condom, it must also, surely, be equally morally right for a female prostitute to do so. And if that applies to a prostitute, the same must logically apply to anyone who visits a prostitute. Moreover, the very fact that the head of the Catholic Church has admitted the possibility of exceptions to its blanket prohibition on contraception could prove to be a revolutionary moment – and one that could end up defining Benedict's papacy.

There is a great deal of damage to be undone. The Catholic Church's record when it comes to the prevention of HIV/Aids has been quite shameful. The Vatican's traditional hard-line stance on contraception has led its representatives to embrace some dangerous pseudo-science over the years. The late cardinal John O'Connor of New York branded the idea that condoms could stop the spread of Aids as "the big lie". In 2003, one of the Vatican's most senior cardinals, Alfonso Lopez Trujillo, stated in an interview with Panorama that HIV could pass through "tiny holes" in condoms.

That scientific nonsense sent a deeply destructive message. Advising Catholics not to use condoms was bad enough. But to suggest that they do not work in preventing the transmission of HIV was irresponsibility of the highest order. Even the present Pope argued on a visit to Cameroon last year that the use of condoms could endanger public health and increase the problem of HIV.

In that depressing context of denial and misinformation, these signs of fresh thinking from the Vatican must be regarded as progress. The Pope's words have certainly been welcomed by UNAIDS and other health groups attempting to stop the spread of the infection in sub-Saharan Africa where some 6,500 people die each day from the disease.

There is some uncertainty over whether the Pope intended his remarks to signal a major break with traditional Catholic teaching. Some infer from the lack of Vatican fanfare that accompanied the release of the Pope's words that it was something of an accident. But however it came about, the world should be pleased that the Catholic Church has, finally, opened its doors to the light of reason on Aids.