Roman Catholic teaching rejects artificial methods of birth control. Pope Pius XI maintained, in his 1930 encyclical letter Casti Connubii (“Of Chaste Marriage”), that contraception was “an offence against the law of God and nature”. There is little scope for fruitful debate on this point - not because the issue is trivial, but because it is about metaphysics rather than morals. You either accept the teaching or you do not.

But statements about the effects of contraception are different. While travelling to Cameroon on his first trip to Africa, Pope Benedict XVI yesterday declared that Aids “cannot be overcome through the distribution of condoms, which even aggravates the problems”. The Pope's comment is a testable proposition that has immense implications for human health and welfare. And the medical evidence is overwhelming: his claim is absurd and irresponsible.

The Church's position on the virtues of chastity outside marriage, and fidelity and natural family planning within it, has consistency but it is inadequate to the gravity of the epidemic that afflicts Africa. The United Nations estimates that worldwide there are 33 million adults and children living with the HIV virus - of whom two thirds are in sub-Saharan Africa. While the position is stabilising in some African countries, such as Malawi, others report an increase in new infections. In South Africa, where the former President Thabo Mbeki rejected the scientific consensus that HIV is the cause of Aids, almost 20 per cent of the population have HIV.

There is no cure for Aids. The Church, through its hospitals, hospices and orphanages on many continents, has provided compassion and selfless care for Aids sufferers. But it has opposed the most reliable means of preventing Aids: the distribution of condoms and education in safe sex. Africa has suffered as a result.

In 2000, the Apostolic Nuncio in Uganda contradicted the country's Vice-President and called for young people to ignore official advice on the use of condoms. That is not a pro-life position. It is a guaranteed prescription for more funerals. And the Church's hostility to Aids prevention has extended even to rank, crank pseudoscience. In 2003, Cardinal Alfonso López Trujillo, president of the Vatican's Pontifical Council for the Family, maintained in an interview with the BBC that HIV could pass through condoms.

The Aids epidemic in Africa involves terrible statistics of avoidable death and suffering. But perhaps the greatest tragedy is that medical experts know what works. In the 1990s, Senegal had great success in public health initiatives to tackle Aids, which were supported by Muslim and Christian leaders. Yet the Vatican continues to hamper anti-Aids programmes in two ways. It tries to prevent Catholic medical facilities from distributing condoms, and it argues mistakenly against the demonstrated effectiveness of condoms in reducing rates of infection.

Pope Benedict has shown deft judgement on some issues, including his welcoming of scientific discovery. But many devout Catholics are convinced that the Vatican must amend its position on Aids. They are right. The scientific process of observation and evidence demonstrates the effectiveness of condoms in preventing Aids. And as Matthew's Gospel records Jesus as saying: “I will have mercy, and not sacrifice.”