The Equality Commission of Northern Ireland (ECNI) is supporting Gareth Lee. It launched the case against Ashers bakery after it refused to make Mr Lee a cake supporting gay marriage.
Aidan O'Neill QC, who is working for the Christian Institute that is supporting Ashers Bakery, said if the discrimination case rules in Mr Lee's favour, it could mean Muslims and atheists will have to make products which go against their beliefs.
Mr O'Neill provided several examples, such as a Muslim printer having to publish a cartoon of the Prophet Muhammad, or a lesbian T-shirt printer having to print shirts saying that gay marriage is an "abomination".
He told the Christian Institute: "This is a truly alarming case with far-reaching implications for freedom of speech. It's wrong to force people to use their creative skills to promote a cause they fundamentally disagree with.
"This is about taking issue with the message not the messenger. It's about principles not personalities.
"The ECNI say it's about discrimination. They are correct as it is Ashers which is being discriminated against for the Christian views of the owners.
"Instead of promoting equality, the ECNI want to make people who believe in traditional marriage second class citizens."
Mr Lee is seeking £500 compensation "for injury to feeling, loss and damage sustained by him as a result of this refusal" and an official declaration that he was discriminated against, according to Mr O'Neill.