The ordination of former Anglican bishops as Roman Catholic priests could fundamentally change the Church they leave behind.
The ordination of Keith Newton, Andrew Burnham and John Broadhurst by Archbishop Vincent Nichols signals the inauguration of a special section for former Anglicans - known as the "Ordinariate" - in the Roman Catholic Church.
It was set up by Pope Benedict XVI as a home for traditionalist Anglicans unhappy in the Church of England, and especially with its decision to introduce women bishops.
The Ordinariate will welcome married priests, although they will not be able to become bishops, and it will preserve some Anglican traditions and practices.
As bishops, the three clerics supervised parishes that opted out of contact with women priests.
Like some other traditionalist clergy on the Catholic wing of the Church of England, they don't believe sufficient provision is being made for traditionalists to avoid coming under the jurisdiction of women.
Father Newton has estimated that some 50 other Anglican clergy might join in the coming months, and a couple of dozen parish groups, but many seem likely to wait to see how the Ordinariate develops.
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Speaking last November, Father Newton said he did not mind giving up his former status as a bishop, but acknowledged that joining the new grouping would bring uncertainties.
"There are sacrifices to do with things like where I'm going to live, where I'm going to work, what I'm going to do, how I'm going to be paid, so I'm taking a bit of a step into the dark, a step of faith. But I do that with some joy really."
Clergy have been warned that they will usually need to find alternative forms of funding when they leave their Church of England stipends and pension plans.
The Roman Catholic Church in England and Wales has set aside £250,000 to help fund the Ordinariate, and has said it will have to find other charitable funding.
Clergy - who are unlikely to take their entire congregations with them - might have to take jobs to support themselves and their families.
They will not be allowed to take their Church buildings with them into the new grouping, and it seems unlikely that many will be allowed to share them.
Some in what is a relatively liberal Roman Catholic Church in England and Wales, might resent the influx of quite conservative clergy who are sometimes thought of as being "more Catholic than the Catholics".
Their new organisation, headed by an "Ordinary", rather like a diocesan bishop, will be spread across England and Wales.
Many traditionalists on the Catholic wing of Anglicanism oppose their colleagues' conversion, warning that it will weaken the Church of England as a broad Church able to balance its Protestant and Catholic traditions.
Prebendary David Houlding belongs to the Catholic Group on the Church of England Synod, and regards the ordination with sadness and anger.
His anger is directed partly at his own Church, but he believes converting to Catholicism is premature.
"The Church of England hasn't finally settled what sort of provision [to operate outside the supervision of women bishops] we are going to get," he said.
"There's more work to do, we haven't reached a satisfactory conclusion, there's no certainty that the legislation will go through as it stands."
Mr Houlding regards the Church of England as the continuing "Catholic" Church in England, albeit one reformed after the break with Rome 450 years ago.
He fears that a long-maintained balance will be lost, not just between its Catholic and Protestant wings, but between its liberal and traditionalist elements.
In short he, and others like him, worry that it's becoming a more liberal and more Protestant Church, less able to fulfil its traditional role in serving the whole theological and social spectrum in England.
Father Newton's view is not dissimilar, even if he has come to different conclusions about how to respond to it.
"I think in recent years we have gone much towards a Protestant understanding of the Church..." he said.
"I think there are questions as to whether it can really claim to be part of the one holy and apostolic Church. It seems to have... made changes to holy orders (ordaining women clergy) that the rest of the (universal) Church has advised us not to make.
"I think a Catholic understanding is no longer credible in the Church of England."
Father Newton insists that his conversion to Catholicism and membership of the Ordinariate is not solely to do with the ordination of women, but about maintaining "unity" at a time when he sees the Church of England departing from tradition.
More ordinations of former Anglican clergy as Catholic priests are due to take place just before Easter.
There are few signs of a mass exodus of Anglicans at the moment, but Mr Houlding, for one, fears that Pope Benedict has opened a door in the Church of England, that will in perpetuity encourage unhappy traditionalists to leave rather than fight their corner.
But Father Newton questions how far the "marginalised" Catholic wing of the Church can any more "dictate to a larger group what is right for them".
"We've felt for some time that Anglo Catholics and Evangelicals (some of whom also oppose women bishops) have been holding the Church back from what it wants to do.
"You can't have a Church that believes in women bishops and doesn't believe in women bishops."
Supporters of the Church of England's status as the established, official state Church, see its long balancing act between opposing factions as vital to its survival in its present form, and the benefit they believe that brings to society at large.
There will be many who wonder anxiously how far the ordinations at Westminster Cathedral could undermine it.