Dozens of people demonstrated against the screening of 'Once Gay: Matthew and Friends', which tells the story of former X-Factor Malta contestant Matthew Grech.
They say the movie promotes so-called gay conversion therapy - a claim denied by film makers at the Core Issues Trust.
Dr Mike Davidson, who heads up the Christian group, said it does not provide the practice but instead offers "standard psychotherapeutic and counselling approaches that explores sexual fluidity...".
In a statement, he added: "Some people, for whatever reason, are just not happy with [being gay] and want to move away from it.
"Clearly, it is the responsibility of anyone working in this area to make sure that they are there of their own accord."
Matthew is an ex-gay from Malta who found Jesus. For telling his story he has been branded 'homophobic' and 'hateful.'— Christian Concern (@CConcern) February 13, 2019
"We are being compared to people who do electric shocks. People are buying into the biggest fabrication of the 21st century – 'conversion therapy'." #OnceGay pic.twitter.com/gMvuXCe3FL
A crowd gathered outside west Belfast's Townsend Presbyterian Church - the chosen venue for Thursday's screening.
Among them was John O'Doherty from The Rainbow Project in Northern Ireland, who told Q Radio the film sent the message that "it is not OK to be gay".
"That message causes our community harm, it has led to lose of life... it has broken up families..."
.@johndoc240 @TRPNI at protest outside Townsend St. Presbyterian Church Belfast screening of 'Once Gay - Matthew & Friends.' More from both sides on air am. @AllianceLGBT @Micky_Murray @allianceparty @GreenPartyNI pic.twitter.com/pXaZmM5UKT— Q Radio News (@qnewsdesk) February 14, 2019
A spokesperson for the Presbyterian Church in Ireland said it "reject[s] homophobia in all its forms."
A statement continued: "The church recognises that 'conversion therapy' is both an emotive term and is defined differently by different people.
"However, two things are very clear. Firstly, no 'therapy' of any kind should be undertaken against a person's will.
"Secondly, where a person chooses to seek help from a counsellor, then it is legitimate for such help to be provided."
The lesbian, gay, bisexual and transgender rights group Stonewall defines 'gay conversion', or 'cure', therapy as "any form of treatment or psychotherapy which aims to reduce or stop same-sex attraction or to suppress a person's gender identity".
The UK government announced plans last summer to ban gay conversion therapies.
Stay up to date with the latest news stories from a Christian perspective. Sign up to our daily newsletter and receive more stories like this straight to your inbox every morning.