The Roman Catholic Church has declared August 26 as National Marriage Sunday and is calling on politicians to “sustain rather than subvert marriage”.
The letter, which will be read in all of Scotland’s 500 Catholic parishes, will urge followers to continue to act against efforts to “redefine” marriage.
The Scottish Government has said it is right to introduce same-sex marriages, but has stressed no clergy would be forced to carry them out.
Cardinal Keith O’Brien, the leader of the Church in Scotland, last week broke off discussions on the issue with First Minister Alex Salmond.
In a strongly worded message, the letter will highlight the church’s “deep disappointment that the Scottish Government has decided to redefine marriage and legislate for same sex marriage.”
It will also announce the launch of a National Commission for Marriage and the Family to co-ordinate a campaign against gay marriage.
Cardinal O’Brien, who has described gay marriage as a “grotesque subversion of a universally accepted human right”, said: “The Church’s teaching on marriage is unequivocal, it is uniquely, the union of a man and a woman and it is wrong that governments, politicians or parliaments should seek to alter or destroy that reality.”
He added: “With this letter we will announce the creation of a National Commission for Marriage and the Family, a body which will be charged with promoting the true nature of marriage, it will develop an online presence and produce materials and organise events which will help Catholic families to support and sustain marriage.
“While we pray that our elected leaders will sustain rather than subvert marriage, we promise to continue to do everything we can to convince them that redefining marriage would be wrong for society.”
The Scottish government has pledged to bring forward a Bill on the issue later this year, and has indicated the earliest ceremonies could take place by the start of 2015.