Twenty Australian faith leaders have signed a letter urging citizens of the country to declare their support for same sex-marriage to two federal parliamentary inquiries on the issue.

The move follows an incident in which six Catholic bishops in the state of Victoria wrote to their parishioners to tell them supporting gay couples marrying would be a “grave mistake” and would undermine the institution of marriage.

Signatories of the pro-gay marriage letter, organised by gay marriage campaigner group, Australian Marriage Equality, include the Sydney Uniting Church minister Bill Crews, the Baptist minister Mike Hercock, the Metropolitan Community Church pastor Karl Hand and the Outreach Ministries church fellowship senior pastor Shane Anderson.

A line from the letter reads: “As clergy from various different faiths and denominations in Australia, we believe marriage is a fundamental institution in our society. It fosters greater commitment between partners, provides children with a sense of security and stability, and strengthens ties with families and communities.”

Three private members bills to legalise marriage between gay people are set to go before federal Parliament. Submissions to the inquiry being held by the Senate legal and constitutional affairs committee close today, while submissions to the inquiry being held by the House of Representatives social policy and legal affairs committee close on 20 April.

Currently, the bills look set to be defeated by their Labor opponents, including the Prime Minister, Julia Gillard – and the Coalition. Labor MPs are allowed a conscience vote on the issue, unlike Coalition MPs. Prime Minister Gillard is a long-standing opponent of gay marriage, despite the fact the majority of her party support it.