The Vatican this week included the destruction of embryos on a new list of mortal sins - a particularly pertinent addition given that the Catholic Church in England and Wales is battling controversial new legislation in this very area.
By Clare Murphy
Health reporter, BBC News website
As the Human Fertilisation and Embryology Bill approaches parliament, the campaign is gearing up.
Catholics are being urged to write to their MPs about the bill
Sunday masses across the country end with a plea for parishioners to write to their MPs with a list of reservations about a bill whose remit ranges from allowing same-sex couples to become joint legal parents of IVF babies to creating human-animal hybrid embryos for research.
The message the Church desperately wants to convey to its congregation is that it is not too late to have an impact upon one of the most controversial pieces of legislation to come before the House of Commons this year.
The mammoth bill with numerous clauses is designed to bring the 1990 regulatory framework for fertility treatment and embryo research in line with some of the advances in science - and changes in attitudes - we have seen in recent years.
Its supporters say it will entrench Britain at the forefront of research which holds out the promise of new treatments for a range of debilitating diseases, including Alzheimer's and Parkinson's, using embryonic stem cells.
And it will save lives, they argue, by for instance enshrining the rights of parents to create so-called "saviour siblings" by selecting embryos who will be a genetic match for a sick brother or sister and thus able to offer potentially life-saving stem cell donations.
But the Catholics argue that the price is too high: any procedure which involves selecting embryos and destroying those which are unwanted or which have been used for research - for whatever end - is an affront to human life and dignity.
But there is also a sense of pragmatism within the Catholic camp.
In an ideal world, there would be no bill: there would be no assisted conception to regulate, no embryos created outside the womb, as any form of reproduction that does not involve sex between a man and a woman is an anathema.
But IVF has rapidly become a fact of life, and many Catholics accept that - and even make use of it.
"You do work within the terms of the debate, and I would be the first to accept that the 1990 legislation really needed updating," says David Jones, professor of bioethics at the Catholic St Mary's University College.
"There are positive aspects to this bill: it prohibits sex selection for purely social reasons, which is a very good start, although we'd like to see that extended to preventing deselection on the grounds of disability too.
"It also ensures that donor conceived children have a right to find out about their genetic heritage - and that's something to be applauded too. But of course there are some real problems with it - the human-animal hybrids are something which many people feel deep disquiet about - and not just the Catholics."
It is because the Catholic Church feels it is not alone in many of its reservations that it is pushing for the creation of a national panel to discuss these issues which would include representatives from across the religious, scientific and lay spectrums.
Natallie Evans was not allowed to use her embryos because her former partner refused consent
This would offer a transparent forum for ethical debate that they say the main body involved in this area - the Human Fertilisation and Embryology Authority - is simply not in the position to provide.
And there are indeed areas where beliefs from apparently different ends of the spectrum collide.
There is a major drive for instance within the scientific community to reduce the number of multiple births from IVF - perhaps by producing better quality embryos to start with, while Catholics in principle would be keen on any development which reduced the number of embryos created - and thus the number discarded.
At the same time, there is often immense sympathy from various quarters for women who have created embryos with a former partner and want them implanted, but they end up being discarded as he refuses consent.
His consent remains essential under the terms of the new bill, but it is something the Church - notwithstanding its emphasis on the importance of the father - would be keen to see changed as part of their philosophy that no embryo should be simply "thrown away".
The key question at present is whether the government will allow its MPs a free vote on the bill.
Currently they have been told they may abstain, but not vote against it. Last week more than 100 academics wrote to The Times urging the government not to erode the precedent of "conscience votes" on such controversial legislation.
They wrote that although they did not hold a common position on the bill, they all believed that votes on amendments should not be whipped. MPs from both sides of the debate have also tabled a motion urging a free vote.
Gaining this remains the Catholics' greatest hope.
"MPs of all parties should have the courage to vote against the bill, and in favour of amendments to remove its most destructive aspects," says Helen Watt, director of the Linacre Centre, which studies health issues from a Catholic perspective.
And regardless of their reservations about the bill, many are hoping to use it as a springboard for a fresh debate on abortion - which strictly speaking the legislation has nothing to do with.
MPs opposed to terminations will table amendments to reduce the upper time limit from 24 weeks to perhaps as little as 13.