Pope Francis warned on Saturday of a "global war" against traditional marriage and the family, saying both were under attack from gender theory and divorce.
Francis made his comments in an impromptu response to a question at a meeting of the small Catholic community in the ex-Soviet republic of Georgia.
"You mentioned a great enemy of marriage: gender theory," the Pope said in response to a woman who had asked about it being taught in schools.
He did not elaborate.
Gender theory is broadly the concept that while a person may be biologically male or female, they have the right to identify themselves as male, female, both or neither.
"Today, there is a global war out to destroy marriage," Francis said. "Not with weapons but with ideas...we have to defend ourselves from ideological colonisation."
The Pope has used the phrase "ideological colonisation" in the past to denounce what he says are attempts by rich countries to link development aid to the acceptance of social policies such as those allowing gay marriage and contraception.
Francis, who has been more accepting of homosexuals than his predecessors but opposes gay marriage, also appeared to be referring to it when he said "marriage is the most beautiful thing that God has created" adding that the Bible says God created man and woman to become one flesh.
In the same answer, he said the growing acceptance of divorce was another threat to the family.
Francis said Mass yesterday for an unusually small crowd of just a few thousand Catholics, a celebration that was further dampened when a delegation from the Orthodox Church stayed away.
Ex-Soviet Georgia is overwhelmingly Orthodox Christian and less than one per cent of the population is Catholic.
Still, organisers had been hoping for a much bigger turnout than the 3,000 people who came to the Mass at a stadium in the capital that has a capacity of 25,000.
It was one of the smallest crowds ever seen at an outdoor papal Mass on Francis' 16 foreign trips so far.
In another setback, a delegation representing the Patriarch of the Georgian Orthodox Church, Ilia II, that the Vatican had expected to come to the worship service, did not show up.
The Georgian Orthodox Church is one of the more conservative in the worldwide Orthodox community and has an extreme right wing that is totally opposed to any dialogue aimed at reunion with the 1.2 billion member Catholic Church.
A small group of right-wing Georgian members have dogged the pope at every stop to protest against the visit, carrying signs reading: "Vatican is a spiritual aggressor" and "Pope, arch-heretic, you are not welcome in Orthodox Georgia."
In an apparent effort to allay their fears, Francis told a meeting of priests and nuns that they should not feel like they had a mission to convert Orthodox worshippers, saying this would be "a great sin".
"Never try to practise proselytism against the Orthodox [Church]. They are our brothers and sisters," he said.
Despite the theological differences between the two Churches, Francis had three cordial meetings with the ailing, 83 year-old Patriarch Ilia.
At the last event in Georgia, he went to the riverside town of Mtskheta, 20 km (12.5 miles) from Tbilisi, to meet Ilia in the 11th-century Svetitskhoveli cathedral, the seat of the Georgian Orthodox Church.
There, Ilia expressed his "deep esteem and fraternal love" for the Pope.
Under Francis, who was elected in 2013, the Vatican has made a concerted effort to improve relations with Orthodox Christians in the hopes of an eventual reunion.
Earlier this year, he held a historic meeting with Kirill, the patriarch of the Kremlin-back Russian Orthodox Church, the largest and most influential in world Orthodoxy.
Francis left today for a day-long stop in overwhelmingly Muslim Azerbaijan before returning to Rome.
