(CNN) A German far-right politician, whose party claims "Islam does not belong to Germany," has converted to Islam and resigned his post.

Arthur Wagner, a leading member of Alternative for Germany (AfD) in the eastern state of Brandenburg, left his position on the party's state executive committee on January 11 citing "personal reasons," according to Andreas Kalbitz, AfD's chairman AfD in the state.

Kalbitz told CNN that he only learned about Wagner's conversion in a telephone conversation with him a few days later.

"I was very surprised," he said. "He has been very active in the Christian wing of the party."

Wagner was first elected as an AfD representative in 2015 and was a member of the state committee responsible for churches and religious communities.

Kalbitz said there was "no pressure from the party" for Wagner to resign. "The party supports freedom of religion," he said.

Wagner did not respond to multiple requests for comment from CNN. He previously told German newspaper Der Tagesspiegel that his conversion was a "private matter" but that his party had not pressured him to step down.

Originally formed as an anti-euro party in 2013, the AfD has more recently campaigned on an anti-immigrant, anti-Islam platform, strongly criticizing the decision to allow more than a million refugees into Germany in 2015. The party came third in last year's federal election, winning 12.6% of the vote.

One of the AfD's key's principles is that "Islam does not belong to Germany." It is one of four doctrines stated on the homepage of the AfD's Brandenburg branch.

Speaking at a press conference a week before the federal elections, then co-chair of the AfD Alexander Gauland said that orthodox Islam is "incompatible" with "the principles of the modern, secular, free and democratic law-bound state."

"The growing Islamization of Germany poses an urgent challenge for its public and state order, cultural identity and the internal peace of our country," Gauland said.

Kalbitz told CNN that while "lots of Muslims in Germany are well integrated, Islam itself is not part of German culture", a point he reiterated in a Facebook post Wednesday.

"We regard Islam as a religious-political doctrine that runs counter to our German cultural identity," he wrote, warning against the development of "Islamic parallel societies" while insisting his party supports freedom of belief.