The Christian Institute has told the Northern Ireland Executive it will "not hesitate" to take legal action if legitimate expressions of religious belief are outlawed under a conversion therapy ban.
LGBT activists want a broad definition of conversion therapy but the Christian Institute warns that common practices like prayer, preaching, pastoral support and even parenting will all be put at risk.
Lawyers for the Christian Institute say in a letter to the Executive's Communities Minster, Deirdre Hargey MLA, that the ban could outlaw "the wrong kind of prayer".
They warn that a badly drafted ban "could inadvertently criminalise those in churches and other faith communities who adhere to traditional beliefs about marriage and gender identity".
"Should any proposals from the Department infringe upon the everyday church activities outlined within the enclosed Opinion our client will not hesitate, where appropriate, to seek a judicial review," the letter reads.
The letter has been penned by leading human rights QC Jason Coppel, who is urging Northern Ireland not to follow the path of the state of Victoria in Australia, which recently introduced a ban despite warnings from Christians that they could be unfairly penalised.
Coppel said that people should be free to question LGBT beliefs, and that Christian views "must be treated by the State with neutrality and impartiality".
"One of the fundamental facets of freedom of religion or belief is the right of a religion to determine its own beliefs and practices, the legitimacy of which should not be questioned by the state," he said.
The Christian Institute's Ciarán Kelly said: "Protecting people from dangerous medical practices is one thing but banning preaching, prayer and pastoral care is quite different.
"It would be as tyrannical as it would be unworkable. It is not up to the police, prosecutors or the courts to decide which kinds of prayer are acceptable
"Jason Coppel's advice is quite clear. Christian beliefs on sexuality are protected by human rights law. They may not be fashionable but that doesn't mean you can outlaw them.
"It is shocking to see activists trying to weaponise a 'harm' narrative to justify oppressing biblically faithful churches."