Opposition Leader Bill Shorten has delivered a speech to the Australian Christian Lobby, telling them he is a Christian who believes in marriage equality.

“I believe in God and I believe in marriage equality,” he told the ACL national conference in Canberra.

“I’m a Christian and a supporter of marriage equality under the law.”

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Religion should never be used as an instrument of division or exclusion, he said.

“I believe our current law does exclude some individuals it says to them that your relationship is not equally valued by the state, that your love is less equal under the law.

“We currently have a law that discriminates against adult couples on the basis of who they love.”

Mr Shorten read from the scriptures and said he couldn’t remain silent about those who said marriage equality was the first step on the road to polygamy and bigamy and bestiality.

The ACL has campaigned strongly to prevent the passing of marriage equality laws.

Mr Shorten said in his speech that he knew some of his policy ideas wouldn’t win acceptance among members, but discrimination couldn’t be justified by faith.

Liberal Democrats senator David Leyonhjelm is working on a new bill to allow same-sex marriage, but won’t introduce it until he’s reasonably confident the numbers are there to pass it.

The Liberal Party is yet to decide whether to allow a conscience vote on the matter, but Labor MPs have a free vote.

Other speakers at the conference include Liberal frontbencher Michaelia Cash, Labor frontbencher Shayne Neumann and lawyer Roger Kiska from the conservative Christian non-profit body Alliance Defending Freedom.

Despite Mr Shorten’s support of same sex marriage, equality advocates criticised him for giving the ACL credibility.

Equal Marriage Rights Australia said Mr Shorten’s attendance was hypocritical after Labor’s motion against Liberal politicians attending the “extremely anti-gay” World Families Congress in August.

“For the Australian Labor Party to now turn around and say that it’s ok for Labor party leader Bill Shorten to attend the extremely anti-gay Australian Christian Lobby conference is completely outrageous and extremely hypocritical,” spokesman Ben Cooper said in a statement.

LGBT equality movement All Out said by attending the event Mr Shorten had given the ACL credibility, instead of treating both sides of the debate fairly.

“It’s important to try to change people’s minds, but if that is the case in this instance we’d also like Bill Shorten and Prime Minister Tony Abbott turning up at the premier events of the LGBT community in Australia,” spokeswoman Hayley Conway told AAP.