More than 250 religious leaders in Illinois have signed an open letter in support of same-sex marriage, which the legislature is likely to take up in January. “We dedicate our lives to fostering faith and compassion, and we work daily to promote justice and fairness for all,” the leaders wrote in the letter, which was released Sunday. “Standing on these beliefs, we think that it is morally just to grant equal opportunities and responsibilities to loving, committed same-sex couples. “There can be no justification,” they continued, “for the law treating people differently on the basis of sexual orientation or gender identity.” This is not the first time members of the clergy have endorsed same-sex marriage, but the public nature of the letter and the number of signatures made it an especially strong statement. The timing is also significant: State Senator Heather A. Steans and State Representative Greg Harris, both Democrats, plan to introduce a bill next month to legalize same-sex marriage. Ms. Steans said they would not put the legislation, the Religious Freedom and Marriage Fairness Act, up for a vote unless they were confident it would pass. She added that the Senate, at least, was “definitely within striking distance” of the 30 votes needed for passage and that she hoped the letter would help persuade undecided legislators to support the bill.