Cardinal Francis George, the leader of the Roman Catholic Church in Chicago, warned this week that gay marriage proponents are on the “wrong side of nature,” a jab at the those who say opponents are on the “wrong side of history.”

In an op-ed titled American Freedom and the Catholic Church, George warned that marriage equality is forcing the Catholic Church to “not act as Catholic.”

“We are afraid that the institutions that perform the works of mercy that have been integral to the church's mission for centuries will be forced to become, effectively, government institutions, given permission to exist only if they do not act as Catholic. At stake are Catholic hospitals, Catholic universities and Catholic social services, precisely as Catholic,” he wrote.

George claimed that “the imposition of a definition of marriage that destroys the natural meaning of marital union” is a test case for religious liberty.

“The law now holds that men and women are interchangeable in marriage, as if children did not need both a mother and a father to be born and raised with some security. These are laws that mark societies in decline, demographically as well as morally.”

Americans should be “concerned that we are on the wrong side of what nature teaches us and therefore, at least in the long run, headed for historical failure as a society,” he added.

George strongly opposed passage of Illinois' law allowing gay and lesbian couples to marry.