Wisdom 18:6

By the merit of Egypt, they endured the punishments of darkness, who, blinded by the obscurity of envy, wished to keep the people of God, to whom the light of the divine law was to be handed down, in their prison, and although they intended not even to spare their infants, by the judgment of God, when one son, that is, Moses, was rescued from death by water, a multitude of adversaries was drowned in mighty water through his ministry. In the same way, now the spiritual enemies who strive to destroy the assembly of the faithful, when the elect of God are freed from their enemies through the water of baptism, only those enemies, together with their king the Devil, are overwhelmed in those very waters by the power of Christ. The reason for this mystery was both foreshadowed long ago in the deeds of the ancients, and is now fulfilled through the coming of Christ.

For that: for, stands for but: night, namely the night on which the firstborn were slain, was known beforehand by our fathers, namely Abraham and others: whence Genesis 15: "Know assuredly that your seed shall be a stranger in a land not its own" etc.; or by the Israelites themselves in Egypt, with Moses foretelling it to them, as is clear from Exodus 12. So that truly, that is, with certainty, knowing in what oaths they trusted, that is, how firm were God's promises; Genesis 22: "By myself I have sworn, says the Lord"; likewise Hebrews 6: "That by two immutable things" etc.; they might be of better courage, namely in awaiting the fulfillment of the oath.