The Vatican stepped up its assault on marriage equality, as Pope Benedict XVI said in his Christmas message that same-sex marriage is destroying the very foundations of the family and what it means to be human.

In his annual Christmas address to Vatican officials — one of his most high profile speeches of the year — the Pope said that a person’s gender identity is God-given and unchangeable, and as a result, he sees same-sex marriage as a “manipulation of nature.”

“There is no denying the crisis that threatens it (the family) to its foundations — especially in the Western world,” the Pope said. “When such commitment is repudiated, the key figures of human existence likewise vanish: father, mother, child — essential elements of the experience of being human are lost.”

In the speech, the Pope denounced what he described as people manipulating their God-given identities to suit their sexual “choice” – and destroying the very “essence of the human creature” in the process.

“People dispute the idea that they have a nature, given to them by their bodily identity, that serves as a defining element of the human being,” he said. “They deny their nature and decide that it is not something previously given to them, but that they make it for themselves. The manipulation of nature, which we deplore today where our environment is concerned, now becomes man’s fundamental choice where he himself is concerned.”

He has previously said that gay marriage, like abortion and euthanasia, is a threat to world peace.

The Pope’s Christmas message comes amid mounting concern by the Vatican that the Catholic Church’s teachings on homosexuality are falling on deaf ears, with pro-gay marriage movements gaining ground most recently in the U.S., France and Great Britain.