Koreans have their own variety of cult-y home-brew blend of Christian churches that can give Mormons a run for their money if we're talking crazy.
There's the Unification Church, which has been very active in the US as well:
And there's also the Shincheonji Church:
(I know it's Vice, but it's Vice World News, at least - which is miles better than their western counterpart at this point)
Lots of other minor, fringe Christian cults too but yeah, when a Korean says they're Christian - that may more often than not be a red flag for crazy shit depending on which "christian" sect they belong to and strongly identify with.