The Catholic Church is to continue its drive against plans to legalise gay marriage with a letter from the Archbishop of Westminster calling on Catholics to oppose same-sex weddings, which will be read out at every mass in England and Wales this Sunday.
The letter, signed by Vincent Nichols and his colleague the Archbishop of Southwark, Peter Smith, comes just days after Cardinal Keith O'Brien – the most senior Catholic in Britain – described gay marriage as "grotesque".
Archbishops Nichols and Smith use less shrill language in their pastoral letter but call on Catholics to oppose government plans to allow gay men and women to marry in secular ceremonies. The letter will be read out at more than 2,500 pulpits in what church leaders hope will spur lay Catholics into active opposition against gay marriage.
David Cameron has signalled his intention to legalise gay marriage and later this month the Government will begin a public consultation on the issue. Church leaders have set up the Coalition for Marriage, a pressure group that intends to lobby against any equalisation of Britain's marriage laws.
Calling on Catholics to support such an initiative, the Archbishops wrote: "Changing the legal definition of marriage would be a profoundly radical step. Its consequences should be taken seriously."
Pro-gay marriage groups hit back. Terry Sanderson, head of the National Secular Society, said: "What the Government is actually proposing is entirely secular in nature, they are only proposing changes to civil marriage. For all their double talk about it being about religious freedom, homophobia does lie at the heart of it. If you see the sort of language the Pope uses about gay people it's not just a kind of 'we stick by our biblical morals', it's a kind of 'we are repulsed by this'."
His letter to Catholics: What Vincent Nichols wrote... and what he means
Dear Brothers and Sisters in Jesus Christ, this week the Coalition Government is expected to present its consultation paper on the proposed change in the legal definition of marriage so as to open the institution of marriage to same-sex partnerships [1 - what he really meant - see below].
Today we want to put before you the Catholic vision of marriage and the light it casts on the importance of marriage for our society.
The roots of the institution of marriage lie in our nature. Male and female we have been created, and written into our nature is this pattern of complementarity and fertility. This pattern is, of course, affirmed by many other religious traditions. Christian teaching fills out this pattern and reveals its deepest meaning, but neither the Church nor the State has the power to change this fundamental understanding of marriage itself .
Nor is this simply a matter of public opinion. Understood as a lifelong commitment between a man and a woman, and for the creation and upbringing of children, marriage is an expression of our fundamental humanity. Its status in law is the prudent fruit of experience, for the good of the spouses and the good of the family. In this way society esteems the married couple as the source and guardians of the next generation. As an institution marriage is at the foundation of our society.
There are many reasons why people get married. For most couples, there is an instinctive understanding that the stability of a marriage provides the best context for the flourishing of their relationship and for bringing up their children .
Society recognises marriage as an important institution for these same reasons: to enhance stability in society and to respect and support parents in the crucial task of having children and bringing them up as well as possible. The Church starts from this appreciation that marriage is a natural institution, and indeed the Church recognises civil marriage.
The Catholic understanding of marriage, however, raises this to a new level. As the Catechism says:
"The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, by its nature is ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptised persons has been raised by Christ the Lord to the dignity of a sacrament." 
This is a high and noble vision, for marriage is a high and noble vocation. It is not easily followed. But we are sure that Christ is at the heart of marriage, for his presence is a sure gift of the God who is Love, who wants nothing more than for the love of husband and wife to find its fulfilment. So the daily effort that marriage requires, the many ways in which family living breaks and reshapes us, is a sharing in the mission of Christ, that of making visible in the world the creative and forgiving love of God.
The reasons given by our government for wanting to change the definition of marriage are those of equality and discrimination. But our present law does not discriminate unjustly when it requires both a man and a woman for marriage. It simply recognises and protects the distinctive nature of marriage. 
Changing the legal definition of marriage would be a profoundly radical step. Its consequences should be taken seriously now. The law helps to shape and form social and cultural values. A change in the law would gradually and inevitably transform society's understanding of the purpose of marriage. It would reduce it just to the commitment of the two people involved. There would be no recognition of the complementarity of male and female or that marriage is intended for the procreation and education of children.
We have a duty to married people today, and to those who come after us, to do all we can to ensure that the true meaning of marriage is not lost for future generations. 
 Archbishop Vincent Nichols has clearly timed his pastoral letter to capitalise on the recent pronouncements by senior Anglican and Catholic clergy who have declared their opposition to gay marriage. But his language is less shrill than his colleague Cardinal Keith O'Brien, who described gay marriage over the weekend as "grotesque". Instead the Archbishop of Westminster seeks to stir up opposition by reminding Catholics of what the church's official teachings on marriage are.
 For the Catholic Church, marriage essentially two things: the joining of a man and woman for love and for the procreation of children. Until recent decades the emphasis was very much on the latter. The use of the word "nature" will raise some eyebrows, given that homosexuality occurs frequently in the natural world. The Catholic Church equally has no issue with heterosexual couples marrying when they know they can't have children.
 This is where Nichols is at least in agreement with David Cameron. Both take the opinion that marriage creates a more preferable environment for children than single parents. This argument may, however, come to undermine the Catholic position. After all, if you prefer children to be brought up in married families, why stop gay men and women who have children from doing just that?
 The fact that marriage is one of the seven sacraments within the Catholic Church helps us understand why its hierarchy is so determined to keep it exactly as it is. In the eyes of the church, sacraments are "signs of grace", acts that Christ not only approves of but thoroughly encourages. For gay Catholics, however, that makes their inability to marry even more painful as they are denied something the church insists is a spiritually vital rite of passage.
 This is the crux of the Archbishop's argument. Equality, they say, already exists in the form of marriages for heterosexuals and civil partnerships for homosexuals. Creating gay marriage would discriminate against the religious. Yet nowhere in this letter to the Catholic faithful is there an attempt to address that the Government has at no point suggested that gay marriages will be forced on unwilling religious communities.
 This isn't just a lament for traditional marriage or how family values risk being eroded: it's a call to arms. The Catholic leadership is concerned that lay Catholics may be opposed to the idea of gay marriage but will do little to stand up and be counted. What they want is furious lobbying over the coming months as the Government discusses its plans in an attempt to stop the process in its track.