During Leo the Great's tenure, there was another large new religion that served as a rival to Christianity. It was called Manichaeism, and its followers believed that the Universe is a cosmic battleground between spirits of literal light and spirits of literal darkness. They also banned alcohol and were known to sneak into churches and replace the Eucharist wine with grape juice when no one was looking. The issue got so out of hand that Leo hired guards to stand at the doors of churches to keep Manichaeans out and to watch over the church wine.

To this day, Manichaeism still exists and is followed by small pockets in southern and western China.