Australian Christian Lobby head Lyle Shelton has quit the role to pursue a career in politics.
The ABC understands Mr Shelton is joining Cory Bernardi's Australian Conservatives party.
Mr Shelton was a key campaigner for the No case during last year's same-sex marriage debate.
In a video posted on social media, Mr Shelton said he "felt very much drawn to the political side of things, to the partisan side of politics" during the same-sex marriage campaign.
"With that in mind, I've decided to make a big step and to move back to my home state of Queensland," Mr Shelton said.
"I'm not leaving the battle for the values that you and I hold dear, just simply going to a different part of the battlefield."
It is not clear if Mr Shelton will look to run for a federal senate or regional Queensland seat.
"I do believe politics needs Christian influence from the inside and our political parties need good people from the inside," he said.
It is understood Senator Bernardi will join Mr Shelton for an announcement in Queensland tomorrow.
