The Rev. Sun Myung Moon, founder of the church colloquially known as the Moonies, is unconscious and undergoing treatment for pneumonia at a hospital in South Korea, a spokesman said Thursday.
His doctor has given him only a 50% chance of survival, spokesman Ahn Ho-yeol said.
Doctors put the 92-year-old founder of the Unification Church in intensive care Tuesday, where he is breathing through a respirator, the church spokesman said.
Moon felt ill suddenly and was hospitalized in Seoul in "grave condition."
The Unification Church gained fame worldwide for its mass weddings decades ago, including at New York City's Madison Square Garden.
Many met their spouses-to-be for the first time during the ceremony. In addition to weddings in South Korea, couples from various countries took part in the ceremony through satellite hookups.
The controversial Moon, whose church critics compare to a cult, served a federal prison term in the United States for tax evasion.
He was also a strong supporter of Republican politicians including Richard Nixon and Ronald Reagan, said Eileen Barker, a professor at the London School of Economics and Political Science.
He was "virulently anti-Communist," having been imprisoned in North Korea during the Korean War before being freed by the allies, she said.
Moon is the founder of The Washington Times newspaper, which vocally backed Reagan, she said.
In 2010, the newspaper was sold to a group operating on his behalf, according to a statement on the paper's website.
In his later years, his position toward North Korea softened, and he met the late North Korean dictator Kim Il-sung and invested some money in the North.
His followers regard Moon as the messiah who is completing the salvation that Jesus Christ failed to accomplish.
His church says Jesus was divine but he is not God, a position that puts the Unification Church outside the bounds of traditional Christianity.
Different measures are under way in case of the worst-case scenario, according to the church spokesman. He did not elaborate on the measures.
Followers of the church worldwide are praying for his health, he said.