On that evening of your baptism you entered into the outer hall of the Baptistry, and there facing toward the west you heard the command to stretch forth your hand, and as in the presence of Satan, you renounced him.
This figure is found in ancient history. For when Pharaoh, that most cruel and ruthless tyrant, oppressed the free and royal people of the Hebrews, God sent Moses to bring them out of the evil grip of the Egyptians.
Then the doorposts were anointed with the blood of the lamb that the destroyer might flee from the houses which had the sign of the blood. And the Hebrew people were marvellously delivered.
The enemy, however, after their rescue, pursued them, and saw the sea wondrously parted for them. Nevertheless he went on, following in their footsteps and was all at once overwhelmed and engulfed in the Red Sea.
Now turn from the ancient to the recent, from the figure to the reality. There, we have Moses sent from God to Egypt; here, Christ sent by his Father into the world.
There, that Moses might lead forth an oppressed people out of Egypt; here, that Christ might rescue mankind who is overwhelmed with sins.
There, the blood of a lamb was the spell against the destroyer; here, the blood of the unblemished Lamb Jesus Christ is made the charm to scare evil spirits.
There, the tyrant pursued even to the sea that ancient people; and in like manner this daring and shameless spirit, the author of evil, followed you, even to the very streams of salvation. The tyrant of old was drowned in the sea; and this present one disappears in the saving water.
What then did each of you standing up say? “I renounce Satan”, meaning “I fear your power no longer, for Christ has overthrown it, having partaken with me of flesh and blood, that through these he might by death destroy death that I might not for ever be subject to bondage.
“I renounce you, you crafty and most subtle serpent. I renounce you, plotter as you are, who under the guise of friendship brought about disobedience and the apostasy of our first parents. I renounce you, Satan, the source of all wickedness.”
When you renounced Satan, utterly breaking all covenants with him, that ancient connection with hell, there is opened to you the paradise of God, which he planted toward the east, where for his transgression our first father was exiled.
Symbolic of this was your turning from the west to the east, the place of light. Then you were told to say: I believe in the Father, and in the Son, and in the Holy Spirit, and in one baptism of repentance.
Cyril of Jerusalem (c. 313-386): Catechesis 19, On the Mysteries 1:1-4.9; from the Monastic Office of Vigils, Monday of the Second Week in Lent, Year 2.