The law allows religion to be a standard of employment in religious schools, he explained. This allows the school "to control the learning environment and who teaches in it."
"It's pretty clear from the law that the schools have the right and the obligation to stay in control of their curriculum," he said.
According to Destro, the question is not a matter of the teacher's sexual orientation.
"The question is about whether you're bearing sufficient witness to the faith in the course of your teaching, another question entirely."
Even if a teacher is not Catholic, Destro said, school contracts usually specify standards of behavior concerning teachers' roles.
CNA sought comment from the diocese but did not receive a response by deadline.
"The Diocese of Charlotte has not received any paperwork. Typically we don't discuss ongoing litigation," Hains, the diocese's communications director, told the Charlotte Observer. He said diocesan officials had not seen the lawsuit as of Jan. 11.
In January 2015 Hains noted the Church's belief that marriage is a union only of a man and a woman and rejected claims of discrimination.
"He's not being picked on because he's gay. He lost his job as a substitute teacher because he broke a promise because he chose to oppose church teaching, something he promised he would not do."
Billard said that his adherence to Catholic teaching was never part of the employment process.
The lawsuit seeks back pay and benefits, punitive damage, compensatory damages for emotional distress, and a court order blocking the school and Catholic leaders from taking similar actions in the future, "restraining Defendants from engaging in further discriminatory conduct."
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The legal action comes after several years of advocacy against religious freedom protections. The American Civil Liberties Union is receiving funding from groups like the Arcus Foundation for projects to "beat back" religious exemptions, grant listings show.
The foundation, founded by billionaire heir Jon Stryker, is also funding some Catholic dissenting groups. Stryker was a major funder of the effort to redefine civil marriage in the United States.