The Presbyterian Church has written to all Northern Ireland Assembly members (MLAs) stating its staunch opposition to equal marriage.
The assembly is debating a motion on same-sex marriage this afternoon.
The BBC reports, in its letter, the Presbyterian Church said it was “not merely an issue of conscience for Christian people and churches, but a very significant one for the whole of society”.
It said marriage for same-sex couples would “effectively demolish” generations and centuries of societal norms.
“The steady erosion of such values, with minimal debate about the world view replacing them, causes us the very greatest concern,” it added.
The church argued it was not an equality issue because it said all of the significant legal benefits and rights available through marriage are already available through civil partnerships.
Green Party MLA Steven Agnew said gay Christians were being “denied rights and religious freedoms”.
“As the law stands at the moment, a couple without faith can get married in a church while a devoutly religious couple of the same sex cannot,” he said.
“Whether a religious institution performs same-sex marriage ceremonies is a matter for the church involved, not the state.
“This is actually an extension of rights to religious institutions to make their own decisions on this issue.
“Therefore, the law preventing churches performing same-sex marriage ceremonies should be removed and instead legal protection should be given to churches to allow then to determine what they define as marriage,” Mr Agnew said.
The DUP is opposed to any change in the law, while Sinn Fein is in favour.