What does Into the light I command thee mean?

This phrase is part of some exorcism rituals of the Catholic church. Most people only know it from the movies The Exorcist (1973) and Constantine (2005). Similar phrases are used in another 2005 movie, The Exorcism of Emily Rose.

Literally, the exorcist is commanding a demon to show himself. Demonic possessions are hidden, secret, and therefore "in the dark". In showing himself in the light of reality, the demon may be addressed and ultimately expelled from his host. The "light" is also a figurative expression of the Light or power of God.