What made him famous was his fight against cement companies in 2006.
"I found cement companies in Korea imported all kinds of waste from Japan and mixed them with limestone to make cement, which can be very toxic to human health. I argued that's why so many children had atopic problems and brand-new apartment buildings could be so bad for people's health," Choi said in a telephone interview with The Korea Times, Tuesday.
The activist sneaked into ports in Samcheok and Donghae on the east coast to film waste from Japan being unloaded from vessels. He also sneaked into a local cement factory to verify the imported waste being processed and getting mixed with limestone, the main ingredient for cement. Limestone is mixed with metallic oxides and sometimes clay to create cement used to make concrete.
"Instead of clay, iron ore and tin, the companies were putting imported waste into the mix. If cement is made with recommended substances, it would not be very toxic, but because of this cheating, domestic cements were toxic."
He wrote a book disclosing the harm done by made-in-Korean cement and waged campaigns to pressure the government to act.
Although not completely banning cement mixed with recycled waste, the Ministry of Environment adopted guidelines on how much toxic waste was allowed in the cement mix. It also required importing companies to report details of the imported waste materials.
Since 2015, he's been fighting against a local company over building a concrete admixture research center in Yongin, Gyeonggi Province where he is living.
"The research center will handle many cancer-causing materials like acrylamide, methyl alcohol, acrylic acid and cyclohexane. And the center will be right across from an elementary school," Choi said.
In addition, Choi discovered the research center faked a report about toxic waste water production in a document needed to get a building permit from the city government. Choi also raised allegations that the company "lobbied" the local government.
In response, the company sued him for defamation and obstruction of business. The legal fight is still underway at the Supreme Court.
Asked about his motivation, he said, "I see problems around me and can't just stay indifferent." He also said, "My job as a pastor is to save lives. What I am doing is also about saving lives."
What got him into environmental activism was his fight to preserve Seo River in Youngwol County, Gangwon Province in 1999.
"I was spending a quiet time in the county as a new pastor just out of school. The county was trying to build a waste landfill next to the river. I grew up with a river and knew how precious it is." He fought along with residents for two years and got the plan canceled. "When I look back, that changed my destiny for good."
Choi gave heartfelt thanks to his wife. "My job would not be done without my wife's support. I am truly thankful to her." They don't have a child. "If I had one, it would have been difficult to do this because I would have needed to earn money to support the child."
He is now working on a new book about waste management.
"The landfill site for Seoul and the metropolitan area is filling up. Estimates say only three years are left before it is full. The government hasn't even chosen the next site. Waste is a big issue. Wherever you look in Korea, there's waste. The Korean peninsula is getting filled with waste, mostly construction waste. I want people to think about the waste issue (through my book)."