By Robert Pigott
BBC Religious Affairs Correspondent
Pope Benedict XVI is a man of enormous authority.
When he says "don't use condoms - even to prevent the spread of Aids" it has a significant impact among tens, even hundreds of millions of people.
Getting on for a fifth of Africans are Roman Catholic.
The Church has been growing more quickly in Africa than anywhere else, and this is the Pope's first visit there in the four years he has been the spiritual leader of the world's approximately one billion Catholics.
With Africans - 22 million of whom are infected with HIV - hanging on his every word, that made his statement aboard the plane heading to Cameroon this week all the more significant.
The Pope said the "cruel epidemic" should be tackled through fidelity and abstinence rather than condoms, and that "the traditional teaching of the Church has proven to be the only failsafe way to prevent the spread of HIV/Aids".
An awareness of the Pope's huge position of strength has sharpened the criticism of his remarks, by others with a more liberal approach to preventing the transmission of HIV.
Rebecca Hodes, working in South Africa for the Treatment Action Campaign, was among the most trenchant critics.
She described Pope Benedict's remarks as "alienating", "ignorant" and "pernicious".
The Pope says HIV/Aids threatens economies and societies
The Pope is echoing the teaching of his predecessor, Pope John Paul II, whose own advocacy of abstinence was also fiercely criticised, and led to accusations even that he was responsible for the spread of disease.
The Church's case has not been helped in the past when senior figures - including the president of the Vatican's Pontifical Council for the Family, Cardinal Alfonso Lopez Trujillo - have insisted that HIV inevitably passes through holes in the latex from which condoms are made, a claim dismissed by the World Health Organization.
However, the Church's concern about condoms is only part of wider teaching aimed at allowing people to live better, more fulfilled lives.
It believes that encouraging people to use condoms to minimise the worst effects of behaviour that in itself impoverishes their lives is to fail them.
Pope Benedict put it this way not long after he took office in 2005. He told African bishops that contraception was among trends leading to a breakdown in sexual morality.
"It is of great concern that the fabric of African life, its very source of hope and stability, is threatened by divorce, abortion, prostitution, human trafficking and a contraception mentality."
In other words, there is something at stake that is greater even than the fight against Aids - particularly as, in the Church's view, condoms are not as effective as abstinence in combating this deadly infection.
It is not as though Pope Benedict underestimates HIV, acknowledging that "the virus seriously threatens the economic and social stability of the [African] continent".
However, Catholics point out that if a couple, one of whom had the virus, consistently used a condom they would reduce the yearly risk of passing it on to the uninfected partner by about 87% - which they say falls well short of an adequate means of disease prevention.
Uganda used a policy combining abstinence, fidelity and - only if necessary - the use of condoms, to achieve a significant reduction in the spread of HIV.
Even some senior Roman Catholics take a pragmatic view of the use of condoms.
The Belgian Cardinal Goddfried Daneels said in 2004 that using a condom with the intention of stopping disease was morally different from using one to prevent the creation of life.
He said condoms could be the lesser of the two evils.
Father Gerry O'Collins, Emeritus Professor at the Gregorian University in Rome, said the commandment 'thou shalt not kill', "trumps other issues".
The Catholic aid agency Cafod is bound to uphold the official teaching of the Church, and it makes clear that it does not fund or advocate the supply, distribution or promotion of condoms.
However, Cafod also points out that condoms are particularly effective for people such as prostitutes who are at highest risk of infection.
Aid agencies can find that their biggest challenge is trying to overcome cultural objections to using condoms.
It could be that the way Pope Benedict chose to repeat the Church's teaching about contraception, even at a time when Aids is rampant, was also significant.
He could have included the issue in a major homily at a high-profile mass.
The fact that he dealt with it in the plane to Africa, answering questions from journalists, could be seen as a subtle distancing of his message from realities on the ground.