Archbishop Glenn Davies has joined other church and religious leaders in signing an open letter to the Prime Minister against same-sex marriage.

Parliament could take steps to redefine marriage as early as August.

The letter, signed by Dr Davies and the Catholic Archbishop Anthony Fisher as well as 36 other religious leaders including the heads of Pentecostal and orthodox churches as well as senior rabbis from the Jewish community and leaders from both the Sunni and Shia Islamic communities, expresses 'grave concerns' regarding 'Bills that have or will be introduced into the Federal Parliament to change the definition of marriage in Australian law'. 

It says the definition of marriage in the current law (the Commonwealth Marriage Act of1961), which describes it as “the union of a man and a woman, to the exclusion of all others, voluntarily entered into for life” reflects a truth deeply embedded across diverse communities, faiths and cultures.

"To uphold marriage as the mutual love of a man and a woman, open to the gift of children, is not bigotry or prejudice. At many times throughout history, and sadly still today in some places, people with same-sex attraction have suffered injustice. This is to be deplored." the letter says. "We should do more to ensure that our brothers and sisters who are same-sex attracted are treated with the dignity and respect owed to every human being. But this does not require the further deconstruction of marriage as traditionally understood."

The leaders say "Australia’s definition of marriage as a union of a man and a woman is consistent with that of the vast majority of world nations, who represent over 91 per cent of the global population. To date, only 21 of the 193 member states of the United Nations have changed their legal definition of marriage to incorporate same-sex unions."

"For the sake of children and to encourage men and women to commit to one another and to their offspring, marriage between a man and a woman has always been given the special recognition and support of the state." the leaders say.

"Any adult person can love and care for a child. But, as a couple, two persons of the same sex are not able to provide a child with the experience of both mothering and fathering."

The letter says overseas experience has shown redefining marriage will have consequences for everyone 'as the truth about marriage becomes increasingly a truth which cannot be spoken'.

It speaks of 'legal vulnerabilities' for the millions of Australians who do not wish to see marriage redefined and the fact that in many overseas cases, expressing such belief has resulted in vilification and legal punishment of individuals and institutions.

"This violates not only freedom of religion, but also the rights of conscience, belief and association, and the right of parents to educate their children according to their own beliefs. The experience of these countries which have redefined marriage demonstrates that attempts to address these concerns through legislative “exemptions” have proven to be worthless." the leaders say.

"We urge you and your fellow Members of Parliament to uphold the law of the Commonwealth of Australia regarding marriage as the union of a man and a woman and to continue to support the common good of our community by supporting true marriage."

It ends with a request for a meeting with the Prime Minister and MP's to further discuss the issue.