The former Atlanta fire chief fired after voicing biblical beliefs in a book has filed suit against the city. 

Kelvin Cochran, represented by Alliance Defending Freedom (ADF), filed a federal lawsuit against Mayor Kasim Reed and the city of Atlanta on the grounds that he was unlawfully terminated. 

Cochran became fire chief of the southern city in 2008, but briefly left to serve as President Obama's US fire administrator. He returned in 2010 at the behest of Mayor Reed.

In November 2013, Cochran published a book condemning sin and urging men to dedicate their lives to Christ entitled, "Who Told You That You Were Naked?"

According to KTBS, the book called homosexuality a sin, and said that marriage should be between a man and a woman. 

After an outcry from LGBT activists in November 2014, Mayor Reed suspended Cochran for 30 days, ordered the fire chief to complete sensitivity training, and said that an investigation would be launched.

The ADF reported that the investigation found no evidence that Cochran had discriminated against anyone during his employment, and the book was written in the chief's spare time. 

Last month, Cochran was fired from his position, a move that ADF Senior Counsel David Cortman said is illegal. 

"Americans are guaranteed the freedom to live without fear of being fired because of their beliefs and thoughts," he said in a statement.

"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."

Cochran said he was devastated by the termination. 

"To actually lose my childhood-dream-come-true profession – where all of my expectations have been greatly exceeded – because of my faith is staggering," Cochran said. "The very faith that led me to pursue my career has been used to take it from me. 

"All Americans are guaranteed the freedom to hold to their beliefs without the consequences that I have experienced."

Mayor Reed, however, is not budging. 

"I wish Mr Cochran well, but I've tried to make myself very clear," he insisted. "It would take the United States Supreme Court to insist that I hire him back."