Cardinal Keith O'Brien and the bishops of Scotland have written a pastoral letter on marriage that is to be read out in all of Scotland's 500 Catholic parishes on Sunday.
The Catholic Church in Scotland is inaugurating National Marriage Sunday on the same day.
The strongly worded letter stresses that "marriage is a unique lifelong union of a man and a woman".
The bishops make clear their "deep disappointment that the Scottish Government has decided to redefine marriage and legislate for same sex marriage".
Parishioners are thanked for their support in defence of marriage and urged to "continue to act against efforst to redefine it".
Cardinal O'Brien, President of the Bishops' Conference of Scotland, endorsed the letter, saying: "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."
The letter also announces the creation of the National Commission for Marriage and the Family, which the Church will use to promote "the true nature of marriage" and develop resources to support families.
Cardinal O'Brien added: "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."