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Cardinal Francis George said Tuesday that same-sex marriage laws create a "legal fiction" and urged parishioners to fight a bill that would allow gay marriage in Illinois.
"The state has no power to create something that nature itself tells us is impossible," he wrote in the pastoral letter.
The Religious Freedom and Marriage Fairness Act could be introduced by Rep. Greg Harris and Sen. Heather Steans this week when the legislature reconvenes for a lame-duck session. New lawmakers will be sworn in Jan. 9.
The Chicago Democrats say public opinion favoring equal rights for same-sex partners is moving rapidly. Constituents in four states voted in favor of the issue or opposed a ban on it during the November election.
Harris successfully brought civil unions to Illinois in June 2011. He and Steans say a gay marriage law wouldn't interfere with religious beliefs or force places of worship to recognize same-sex marriages.
George said that if lawmakers allow gay marriage in Illinois, they will be "acting against the common good of society."
"We will all have to pretend to accept something that is contrary to the common sense of the human race," he wrote.
But not all leaders of faith agree with the cardinal. In the days before Christmas, more than 200 rabbis and pastors issued a joint letter urging lawmakers to pass the bill. President Barack Obama has also voiced his support of the bill.