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There's a joke told about Cardinal Joseph Ratzinger, prefect of the Vatican's powerful Congregation for the Doctrine of the Faith. He, and two theologians he "disciplined", Leonardo Boff and Hans Kung, are at the pearly gates. Peter says God will see each separately.

Kung goes in first and comes out muttering "how could I have been so wrong?" Boff goes in and soon he too emerges muttering "how could I have been so wrong?"

Cardinal Ratzinger goes in. Hours pass. There are shouts and sounds of weeping. The door swings open. God comes out muttering "how could I have been so wrong?"

Like all good jokes it has the ring of truth about it. There is no doubt that where many are concerned, his Eminence is frequently seen to be more authoritative than God.

A version of that joke is repeated in a new biography Cardinal Ratzinger: the Vatican's Enforcer of the Faith. Written by John Allen, a journalist with the American National Catholic Reporter, it is a fair and unfailingly interesting account of one of the most controversial figures in religion today.

In his almost 20 years with the Congregation for the Doctrine of the Faith (CDF), he has "seen off" liberation theology, helped to declare "closed" the debate on women priests, and more recently put all other Christian denominations and religions in their lowly place. He has silenced theologians, advanced the careers of supporters and banished to outer darkness those who disagreed.

Yet this deeply conservative "enforcer" began his clerical career as one of the bright, liberal stars of Vatican II, where he was personal theologian to Cardinal Joseph Frings of Cologne. How he shifted from such generous youth to rigid old age is the great conundrum.

Joseph Aloysius Ratzinger was born in Bavaria on April 16th 1927, the youngest of three children, to Joseph and Mary (the coincidence is not lost on some), a lower-middle class couple. He a policeman. She a cook.

Hitler came to power six years later and Nazism was to dominate his world until he was 18, though not his life. His family was devoutly Catholic. He came to believe later that only the Catholic Church presented a challenge to Nazism. A unified church, clear on its own core convictions, was all that could stand up to the pressures of a totalitarian state, he felt.

It does not, therefore, take great imagination to understand how he and the Pope could be of such a similar mind. It illustrates, too, how totalitarianism breeds its mirror-image in opponents.

This book points to the events of 1968 as the spur which propelled the Cardinal rightwards. With student revolts throughout the West, including at Tubingen where he was teaching, he feared "mere anarchy" was being loosed upon the world. Since then he has been the champion of a Catholic order.

Also remarkable is the parallel between his career and that of another great liberal/turned conservative, Pope Pius IX. He began his papacy as a breath of fresh air in 1846. That all changed in 1848, a year of revolutions throughout Europe during which Pius was forced into exile. Returning, he revoked all reforms and set his face against the world.

There was much surprise last September when Pius was declared "Blessed", along with Dom Marmion and Pope John XXIII. Rightly or wrongly, Cardinal Ratzinger's hand was seen in what was thought to be a balancing act between the liberal John and the retrenchment of Pius. Both Blessed popes mark out the poles of Cardinal Ratzinger's own life.

Cardinal Ratzinger: The Vatican's Enforcer of the Faith by John Allen is published by Continuum and is distributed in Ire- land by Columba. Price £19.99.