Alliance Defending Freedom is suing the City of Atlanta and its mayor on behalf of the city's former fire chief, Kelvin Cochran.
OneNewsNow and American Family News have reported on the fire chief's firing by Mayor Kasim Reed after Cochran wrote and self-published a book that condemned sexual immorality, including homosexuality.
ADF announced the federal lawsuit at a news conference Wednesday.
The lawsuit comes after ADF filed a complaint with the Equal Employment Opportunity Commission, on behalf of Cochran, last month.
At the press conference, ADF senior counsel David Cortman said the City of Atlanta "is not above the Constitution and federal law. In America, a religious or ideological test cannot be used to fire a public servant."
ADF reported in mid-January that was looking at "legal options" after Cochran was fired by the mayor.