Cardinal Francis George is urging parishioners to fight against gay marriage.
News reports say the cardinal, who heads the Archdiocese of Chicago, wrote a letter to priests encouraging Roman Catholics to contact their legislators about the issue.
Illinois lawmakers are expected to take up legislation approving same-sex marriage as early as this week.
George wrote that such legislation would “be acting against the common good of society.”
"Civil laws that establish 'same sex marriage' create a legal fiction," George wrote, according to news reports. "The State has no power to create something that nature itself tells us is impossible."
In his letter, George tells Catholics they may oppose same-sex marriage, but they should not turn their backs on gay people.
“Good pastoral practice encourages families to accept all their children and not break relationships with them,” the Cardinal reportedly wrote.
There’s been growing support for same-sex marriage in Illinois and across the nation. About 10 states have already legalized the practice, and Maryland held its first gay marriages on Tuesday.
Last week, about 260 clergy members expressed support for legalizing same-sex marriage in Illinois.