The man who wrote last week's Vatican document ruling out same-sex marriage is a soft-spoken Bavarian who was once a liberal but has served as Pope John Paul II's ultra-conservative guardian of Catholic doctrine for more than 20 years.
Cardinal Joseph Ratzinger has been at the Pope's side as head of the Congregation for the Doctrine of the Faith for so long he has been nicknamed "The Enforcer" or the "Panzerkardinal".
Cardinal Ratzinger is regarded as the second most powerful man in the Church.
If anything, he is even more zealous than the Pope, whom he meets every Friday evening, in laying down the law on social or sexual mores.
One joke told in the Vatican has Cardinal Ratzinger arriving in heaven with the church dissidents he has suppressed. The dissenters emerge after meeting God, crying: "How could I have been so wrong?" Then Cardinal Ratzinger goes in to meet the Almighty, there is also wailing and gnashing of teeth -- and God emerges, crying: "How could I have been so wrong?"
So indispensable has Cardinal Ratzinger become that when he turned 75 in April - the usual retirement age for senior churchmen - the Pope asked him to stay on.
The 12-page document last week outlawing same-sex unions and adoptions by homosexuals was first proposed in January, when the Pope urged Catholic politicians to use their votes to "safeguard the family". It was firmed up in March, when the Vatican repeated that those involved in gay marriage had "disordered minds".
Behind the latest broadside lies the Pope's growing fear that this legal and social taboo is rapidly slipping in both the US and Europe, and must be shored up at all costs.
The document was signed by Cardinal Ratzinger on behalf of the Pope, and bore all the hallmarks of his uncompromising battle against alternative ideas. Homosexual acts "go against natural moral law", while to legitimise same-sex unions was "evil" - a word the Vatican does not use lightly.
But Cardinal Ratzinger has dedicated his life to holding the line in a world he regards as lacking in moral fibre. The papal adviser's most withering words are reserved not for the external enemies of the Church but for Catholic liberals who he believes are undermining faith and papal authority by flirting with the "rampant individualism" and "anything goes" culture of the Western world.
The cardinal is a charming and cultured character, with a ready smile and a love of Mozart. He plays the piano well. In his youth in Nazi Germany he was said to be something of a liberal.
Although he was a member of the Hitler Youth, and in 1943 was drafted into the Wehrmacht as a teenager, he came to believe Hitler was the Antichrist. He deserted in 1945 and was held as a prisoner of war by US forces.
He shares the view of the Polish Pope - who as a young man experienced the Nazi occupation of his homeland - that only a united Catholic Church with absolute values can stand against totalitarianism and the temptations of Western materialism.
The cardinal was shocked by the reforms of the Second Vatican Council in the 1960s, when the old certainties gave way to intellectual and theological debate.
After five years as Archbishop of Munich, Cardinal Ratzinger - he was made cardinal in 1977 - was summoned to Rome to be the Pope's enforcer in 1981, swiftly gaining a reputation for ruthlessly quashing dissenters who dared to question papal infallibility or doctrine.
The cardinal has also condemned Buddhism, Hinduism and other Eastern religions as offering false hope through "auto-erotic spirituality", accused the media of exaggerating the extent of the pedophilia scandals in the North American Church and encouraged a return to the Latin Mass.
To the relief of Catholic liberals, he is unlikely to become the next Pope. However, he will play a key role in deciding who that will be. As Dean of the College of Cardinals, he will chair the conclave - and can be expected to do his utmost to ensure the successful candidate is a man in his own conservative image.