Former Atlanta Fire Chief Kelvin Cochran has filed a federal civil rights lawsuit against Mayor Kasim Reed and the city, saying he was terminated last month because of his religion.

Attorneys with faith-based nonprofit Alliance Defending Freedom, who are representing Cochran, filed the suit in a U.S. District Court Wednesday. They say Cochran’s self-authored religious book, which contained controversial passages about homosexuality, cost him his job.

The lawsuit is a separate legal proceeding from a federal discrimination complaint Cochran filed with the Equal Employment Opportunity Commission in January. And it comes on the heels of a Feb. 10 letter sent by U.S. Rep. Barry Loudermilk and several other Republican members of the Georgia delegation to Reed, saying it "appears" the mayor violated "fundamental principles of free speech and religious freedom" when he terminated Cochran.

“It was clear he was fired for his faith, and firing a public employee because he doesn’t pass a religious test is blatantly unlawful,” ADF Senior Counsel David Cortman told reporters on Wednesday. “Think about how dangerous a concept this is, that someone can be fired if their religious beliefs differ from the city’s.”

A spokeswoman for Reed said the city will “vigorously defend” the decision. The mayor has repeatedly said Cochran’s judgment — and not his faith — is why he was terminated after serving a 30-day suspension without pay.

“The totality of his conduct… reflected poor judgment and failure to follow clearly defined work protocols,” spokeswoman Jenna Garland said in a statement Wednesday.

The decorated fire chief was ousted from Reed's administration in early January for what the mayor said was a breach in protocol in Cochran's decision to publish "Who Told You That You Are Naked?" At issue are passages within the 162-page book that describe homosexuality as a "sexual perversion" akin to bestiality.

Many decry the passages as offensive and anti-gay, while others see it as a valid expression of Cochran’s religious beliefs.

Reed maintains that he fired Cochran because the chief didn’t have the required clearance from the city’s ethics office to publish the book, identified himself as the fire chief in its pages and then disobeyed Reed’s request to not speak publicly during the suspension.

And by distributing the book to a small number of employees, Cochran “risked sending a message to his staffers that they were expected to embrace his beliefs,” Garland said Wednesday.

The city found no evidence during its investigation of Cochran that his beliefs have played a role in his leadership.

The chief, who served under former Mayor Shirley Franklin and President Barack Obama, has since become the poster child for a religious liberty bill now under consideration in the Georgia General Assembly.

Cochran said he filed the legal challenge because of its broader implications.

"While I was fired for my faith … it's ultimately not just about Kelvin Cochran," he said. "It's about the future of freedom of speech and freedom of religion in the United States of America."

In the 54-page lawsuit, the ADF asks Reed to reinstate the decorated fire chief and compensate him for lost wages.

Atlanta civil rights attorney Lee Parks believes Reed overstepped his legal authority, and what’s called “prohibition against prior restraint,” in saying Cochran must have received prior approval before publishing the book.

“In other words, the mayor can’t issue a memo to the fire department to say: ‘Before you [use] Twitter, check with me,’” Parks said.

He thinks Reed's argument that Cochran needed approval from the ethics office before pursuing an outside income venture "only works for them if they can show the outside venture violated ethics code. Authoring a book does not."

Parks, who specializes in discrimination and wrongful termination litigation, said Cochran has a compelling case.

“Do I think what Cochran said makes sense? No; it’s despicable,” Parks said. “But does he have a right to say it? Yeah, that’s the whole point of the Constitution.”