SEOUL, South Korea — In 1973, a newspaper advertisement in the South Korean city of Daejeon announced the arrival of “The Messenger From the Spiritual World,” a messiah who embodied the best of Buddhism, Christianity and other religions.

“Religious leaders of all beliefs, come and learn from him!” it read. “Those in distress or afflicted with incurable diseases, come and seek his counsel!”

The Messenger was Won Ja-kyong, one of several aliases used by Choi Tae-min — a man who, more than 20 years after his death, is at the root of a scandal that has roiled this country and sent President Park Geun-hye’s approval rating to the lowest point of any modern South Korean leader, according to Gallup Korea. Tens of thousands of people marched in Seoul, the capital, on Saturday, demanding Ms. Park’s resignation.

The scandal’s primary figure is not Mr. Choi but his daughter, Choi Soon-sil, who has inherited his role as a secretive adviser for the president. This past week, Ms. Choi was arrested and charged with using her influence with Ms. Park to extort large sums from Korean companies.