Some of Australia's most prominent Christian leaders are expected to read out and distribute anti-gay marriage letters at their Sunday services tomorrow.

Statements from Anglican, Catholic and Greek Orthodox leaders have been coordinated ahead of further debate on gay marriage in Parliament next week.

Churchgoers will be urged to contact their Federal MPs to register their opposition to gay marriage.

The head of the Greek Orthodox Church has written to his congregation saying that altering the traditional form of marriage is, in his view, against the sacredness of marriage and of the family as taught by the Christian faith.

The Anglican Archbishop of Sydney, Peter Jensen, has written a letter to all his parishioners saying that for the good of society, the definition of marriage should not be changed.

Archbishop Jensen told Saturday AM he decided to act after the Catholic church told him it would be circulating anti-gay marriage material on Sunday.

"We ought not to feel that the whole matter is being inevitably going in one direction but that we ought to make our voices known so that we make it clear that the Christian faith opposes this for the good of all," he said.

In his letter, Archbishop Jensen says it is beyond the power of parliament to change the definition of marriage.

He says legalising gay marriage would have a "bad impact" on society.

"If for example the Federal Parliament were to change the definition of marriage in the Marriage Act, it would be a symbolic victory for those who think that it is possible for two men or two women to be actually married," he said.

"That symbolic victory would play itself out in the classrooms of the nation; as from then on it would be impossible to teach in the classroom that marriage is exclusively for male and female.

"It would have a bad impact, even though it's attempting the impossible.

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"I would say, following the teaching of the bible, that although friendship and fellowship is a great thing, the sexual relationship between two males or two females is something that's not for our good.

"For those who take that line; which I think is the truth, this will make it very difficult, if not impossible to keep saying that."

He says it is not an issue which can simply disregarded by the Anglican church as it affects all of society.

"You may notice that we're not talking about Anglicans. I'm talking about all of us. This affects you; it affects me. It affects our society," he said.

"My children and your children go to school and - this is just one illustration, let me say - and we will be taught that the sexual relationship, as symbolised in marriage itself, between two men, is just as authentic and just as true and just as good for us as a relationship between a man and a woman.

"Now, all down through human history and in the majority of places in the world today, people don't think that's for the good."