The Vatican's ongoing international crackdown on liberal-minded priests has opened up a new front: nuns.

This week it was revealed that the Vatican’s Congregation for the Doctrine of the Faith has been secretly investigating the Leadership Conference of Women Religious, the largest and most influential group of Catholic nuns in the United States.

It appears that Pope Benedict, who once led the Vatican's powerful doctrinal office, has lost none of his enthusiasm for the job. Once derisively nicknamed 'God's Rottweiler' for his tendency to confront any liberalizing tendency within the church or its theology, his election has been greeted by conservatives in the church as 'the end of the progressive project.'

On Wednesday the powerful Congregation for the Doctrine of the Faith, led by Cardinal William Levada, the former archbishop of San Francisco, announced that a secret investigation into the nuns group had found 'serious doctrinal problems.'

The Vatican claimed that members of the Leadership Conference of Women Religious had challenged church teaching on homosexuality and the male-only priesthood, and had promoted what they called 'radical feminist themes incompatible with the Catholic faith.'

According to a report in The New York Times the nuns were also strongly reprimanded for making statements that 'disagree with or challenge the bishops, who are the church's authentic teachers of faith and morals.'

The implication is quite clear: men (bishops) are the only authentic interpreters of God's will; women (nuns) are not and will no longer be permitted to challenge them on any point.

In 2010 during the protracted negotiation over the Obama administrations historic health care plan, many conservative American bishops opposed it but dozens of nuns signed a statement supporting it, believing it would help the poor. The nun's support proved to be a crucial pillar in the administration's battle over the health care plan.

The Leadership Conference is a diverse association of women’s religious communities with over 1,500 members who represent 80 percent of the Catholic sisters in the United States. But on Wednesday when word of the Vatican’s doctrinal crackdown was announced, the conference's remembers were taken completely by surprise.

'I’m stunned,' Sister Simone Campbell, the executive director of Network, a Catholic social justice lobby founded by sisters, told the New York Times.

Her group was cited in the Vatican document for focusing too much on poverty and economic injustice, while keeping 'silent' on abortion and same-sex marriage.

'I would imagine that it was our health care letter that made them mad,' Sister Campbell said. 'We haven’t violated any teaching, we have just been raising questions and interpreting politics.'

They nun's group has reportedly been given up to five years to revise it's statutes> From now on they must approve of every speaker at the group’s public programs. A handbook they used to facilitate dialogue will no longer be permitted, since the issues it discusses are non-negociable settled doctrine.

It's understood that whilst the Vatican investigated the Leadership Conference, they have also conducted a separate widespread investigation of all women’s religious orders and communities in the United States.

That other crackdown, euphemistically known as a 'visitation,' concluded in 2011 but the results of the investigation have not been published.

The Vatican can still expect pushback from priests and nuns, however. At the weekend Rev. Tim Clark of Seattle’s Our Lady of the Lake Catholic Church received a standing ovation from his parishioners when he announced his parish would not be participating in a campaign to undo the state's recent marriage equality law.

According to Seattle Post the parish is the sixth in Seattle to opt out of a petition drive for Referendum 74, which was endorsed and submitted to the parishes by Archbishop J. Peter Sartain.