"It's really about when the church gets out of step with society and society loses confidence in the church as having a strong moral compass.
"I think the church for many years has been seen as a model that tries to promote good values in society and I think the church has done that well in times in pointing our different issues of justice and promoting honesty and kindness," he said.
"I think that society and science have said that gay people should be treated like anyone else and if the church continues to discriminate the confidence society has in it will diminish."
He said the church could be left behind "as a relic" and needed to change to have society's confidence as a moral body to be listened to.
The vicar has made an application to become a minister with the Presbyterian Church.
He said St Matthews had been a leader in fighting for the rights of sexual minorities, dating back to 1974 when they hosted the country's first congregation explicitly for gay and lesbian Christians.
Anglican Church spokesman Lloyd Ashton said he didn't want to comment because Reverend Cardy "says what he says" and he didn't want to engage with it.
Ray Coster, moderator of the Presbyterian Church of Aotearoa New Zealand, said: "By upholding the historic Christian understanding of marriage as the loving, faithful union of a man and a woman, we are being faithful to what we understand Christ is calling us to as a church."
Glynn Cardy is speaking at the Sea of Faith conference at Somervell Presbyterian Church, at 497 Remuera Rd in Auckland, today from 9am to 4pm.