The worker, named in local media only as Tomasz K, has filed a lawsuit accusing the world's largest furniture retailer of religious discrimination, sparking outrage in the heavily Catholic country where LGBT rights lag well behind the developed world.
Mr Tomasz, a long-term employee at the Krakow store, was sacked after he refused to take down Bible verses he posted in response to a notice on the company's intranet promoting a pro-LGBT event in May.
"I was shaken up," he told Polish national broadcaster TVP Info on Thursday, in comments translated by the Church Militant website. "I've been hired to sell furniture but I'm a Catholic and these aren't my values."
Ikea had asked workers to join in celebrating the International Day against Homophobia, Biphobia and Transphobia on May 16 and "to stand up for the rights of Lesbian, Gay, Bisexual, Transgender plus people of all sexual orientations and gender identities".
The company's head of equality, diversity and integration, Sari Brody, wrote a follow-up post requesting that employees "ask for the transgender person's preferred pronoun (hers, theirs, etc.)" and "engage LGBT+ people in conversations about their partners and families".
Mr Tomasz wrote under the post that "acceptance and promotion of homosexuality and other deviations is a source of scandal", quoting two Bible passages.
"Woe to him through whom scandals come, it would be better for him to tie a millstone around his neck and plunge him in the depths of the sea," (Matthew 18:6) and, "If a man lies with a male as with a woman, both of them have committed an abomination; they shall surely be put to death; their blood is upon them," (Leviticus 20:13).
Mr Tomasz said he made the post because he signed a contract to sell furniture, not implement "so-called LGBT values" or promote "ideological propaganda". "It upset me," he told TVP Info.
"I do not think it was my duty. I put my entry, in which I expressed that it is unacceptable, and quoted two quotations from the Holy Scriptures — about stumbling and about the fact that intercourse between two men is an abomination."
He was summoned into an interview where he was asked to explain himself and told to remove the posts — but he refused. "As a Catholic, I cannot censor God," he said. "I was told there would be consequences."
A few days later he was "informed immediately that Ikea decided to terminate the employment contract, I was supposed to pack right away, empty the cabinet, give up my ID".
On Friday, Ikea responded by saying it had sacked Mr Tomasz for "using quotes from the Old Testament about death and blood in the context of what fate should meet homosexual people" and "expressing his opinion in a way that could affect the rights and dignity of LGBT+ people".
Mr Tomasz is being represented by the conservative legal group Ordo Iuris, which describes its mission as defending the Polish constitution against "various radical ideologies that aggressively question the existing social order".
The group, which has filed a lawsuit in the Krakow District Court seeking compensation and reinstatement for Mr Tomasz, accused Ikea of issuing a "false statement" about his dismissal.
It said Mr Tomasz did not call for violence in posting the quotes but was simply expressing his religious conviction.
"The insinuation contained in the Ikea statement is unacceptable and violates Mr Tomasz's personal rights," Ordo Iuris chairman Jerzy Kwasniewski said, adding it "can be read as motivated by prejudices against Christians".
Mr Kwasniewski said Ikea was attempting to "censor the Holy Bible" by pointing to a quote from the Old Testament as "legally unacceptable and justifying the dismissal of an employee".
On Friday, the country's justice minister Zbigniew Ziobro said he had ordered the prosecutor's office to look into the case to determine whether Ikea had broken any civil or criminal laws. Mr Ziobro described it as "shocking matter" and "absolutely scandalous if confirmed".
On Sunday, a second employee told TVP Info he had resigned as an act of solidarity with Mr Tomasz. "(If Ikea) promotes equality and diversity, why was this Catholic thrown out of work for expressing his opinion?" he said.
According to the Church Militant website, Ikea was threatened with a boycott by Polish Catholics in 2008 after featuring a same-sex couple in its catalogue with the caption "Ian and Steve have no intention of having children" but "enjoy their command centre", an Ikea kitchen.
Earlier this year Ikea produced a special rainbow flag to mark Pride Month but, acknowledging Catholic sensitivities, said the product would not be available in Poland.
In a statement to news.com.au, a spokeswoman for Ikea holding company Ingka Group said while the company welcomed people of all religious backgrounds, "using your religion as a reason for excluding others is considered discrimination".
"At Ingka Group we believe everyone has the right to be treated fairly and be given equal opportunities whatever their gender, sexual orientation and gender identity, age, nationality, religion and/or any other dimension of their identity," she said.
"Inclusion at Ingka Group means respecting our individual differences and creating a safe environment for all. Everyone's views and opinions are welcome with the common goal to build a great place to work.
"This involves taking a stand on which types of behaviours and expressions are in line with our values and ambition for equality and which are not. Personal behaviours and expressions in the workplace cannot intrude on someone else's personal sphere.
"Our human rights and equality policy, supported by our rule of equality and by our code of conduct specifically address lack of tolerance for discrimination and for exclusive behaviour.
"Using your religion background as a reason for excluding others is considered discrimination. While we welcome people of all religious backgrounds and celebrate our differences, this doesn't mean that we endorse exclusive behaviours in the name of religion."
Comment: Also see: Poll finds Americans are growing less tolerant of the 'LGBTQ' movement