Gerhard Ludwig Müller, who has defended liberation theologists and written a book on the poor, will lead key department

Pope Benedict on Monday appointed as the Roman Catholic church's doctrinal watchdog a fellow German with links to liberation theology, the interpretation of Christianity that conservatives have deplored as Marxism with a cross in place of a hammer and sickle.

Gerhard Ludwig Müller, 64, the bishop of Regensburg in Bavaria, is to take over the Congregation for the Doctrine of the Faith, the direct successor of the department created in the 16th century to manage the Inquisition.

It is generally regarded as the second most important department in the Vatican after the Secretariat of State and was for many years headed by Benedict before his election to the papacy.

The congregation's primary role is to keep a beady eye on the writings and teachings of Catholic theologians. But in recent years it has acquired responsibility for dealing with two of the most sensitive issues facing the Vatican – the scandal of clerical sex abuse, and efforts to heal the breach with breakaway ultra-conservative Roman Catholics.

As Cardinal Ratzinger, the pope spent much of his time bringing to heel Latin-American liberation theologists. The late John Paul II repeatedly accused priests inspired by liberation theology of having lost sight of their spiritual mission in their concern for poverty and human rights.

Müller is unquestionably a conservative. He was criticised by the liberal Catholic movement Wir Sind Kirche following his appointment for having considered "the enforcement of church discipline more important than changing obvious wrongs" while bishop of Regensburg. But, like Benedict, Müller has unexpected friends. He was a student of Father Gustavo Gutiérrez, the author of the seminal 1971 work, A Theology of Liberation. And only eight years ago, Müller and Gutiérrez co-authored a book entitled On the Side of the Poor.

The Vatican's new guardian of orthodoxy has on more than one occasion defended liberation theologists, arguing that their outlook is consistent with Catholic teaching. He was chosen to represent the Catholic side in ecumenical talks with the Lutherans. And he can be expected to take an uncompromising line towards the traditionalist Society of St Pius X, which Benedict is trying woo back into the fold.

In a 2009 interview, quoted by Associated Press, he was said to have recommended that the society's bishops should resign and retire "as simple priests to repair a part of the damage the schism has caused".

Müller, who has also been raised to the status of archbishop, will enter a Vatican bitterly divided and alive with intrigue. In May, the pope's butler was arrested, accused of leaking Benedict's correspondence.

© Guardian News and Media 2012