LJUBLJANA, Slovenia (AP) — Slovenians held a referendum Sunday on whether to allow same-sex marriage and become the first former communist nation in Europe to do so.

The vote was forced by conservative groups, backed by the Catholic Church, who seek to overturn a bill that defines marriage as a union between two consenting adults rather than a man and a woman.

The left-leaning Parliament in the small EU state passed a marriage equality amendment in March, but the “Children are At Stake” group has collected 40,000 signatures to challenge the changes before any gay couples were able to marry.

Metka Zevnik, a prominent activist from the group, said that with Christmas approaching, she expected Slovenians to support traditional values.

“Today is a beautiful day … Christmas lies ahead, Christmas Eve too, when Christians gather before the Nativity scene to honor the holy family and the birth of Jesus,” Zevnik said.

Supporters of same-sex marriage — including the left-leaning ruling coalition — have called for Slovenia to join Western European nations that have allowed more gay rights. Conservatives and the right-wing opposition have campaigned on traditional family values, arguing that marriage equality paves the way for gay and lesbian couples to adopt children.

Luka Mesec, from the United Left party that initially put forward the equality bill, insisted that “traditional family will not be affected by this in any way.”

He predicted a “historic day when Slovenia takes a step forward and Slovenians show we are tolerant and open.”

At least 20 percent of the country’s 1.7 million voters must reject the bill for it to be overturned, which means that the outcome will also hinge on turnout. Referendum authorities said that 25 percent of voters cast ballots by the afternoon.

Although Slovenia is considered to be among the most liberal of the ex-communist nations, gay rights remain a contentious topic in the predominantly Roman Catholic nation of 2 million.

Voters in the former Yugoslav republic rejected granting more rights to gay couples in a referendum in 2012 and recent opinion polls have suggested they remain sharply divided over the issue — an illustration of a cultural split within the European Union in which more established western members are rapidly granting new rights to gays, while eastern newcomers entrench conservative attitudes toward the lesbian, gay, bisexual and transgender community.

Igor Zagar, a 55-year-old professor from the capital, Ljubljana, said he voted in favor of marriage equality to “support the secular state and against the interference of the church into political issues.”

Gregor Jerovsek, a 40-year-old mechanic from Ljubljana, said he believed that “the family should not be a field for experimentation.”

“A traditional family should remain the key value of our society,” he said.