Let us proceed to what follows in the text. We have learned through the things examined already that Moses (and he who exalts himself by virtue in keeping with his example), when his soul had been empowered through long application and high and lofty life, and through the illumination which came from above, considered it a loss not to lead his countrymen to the life of freedom.
When he came to them, he implanted in them a more intense desire for freedom by holding out worse sufferings to them. Intending to remove his countrymen from evil, he brought death upon all the firstborn of Egypt. By doing this he laid down for us the principle that it is necessary to destroy utterly the first birth of evil. It is impossible to flee the Egyptian life in any other way.
It does not seem good to me to pass this interpretation by without further contemplation. How would a concept worthy of God be preserved in the description of what happened if one looked only to the history? The Egyptian acts unjustly, and in his place is punished his newborn child, who in his infancy cannot discern what is good and what is not. His life has no experience of evil, for infancy is not capable of passion. He does not know to distinguish between his right hand and his left. The infant lifts his eye only to his mother's nipple, and tears are the sole perceptible signs of his sadness. And if he obtains anything which his nature desires, he signifies his pleasure by smiling. If such a one now pays the penalty of his father's wickedness, where is justice? Where is piety? Where is holiness? Where is Ezekiel, who cries, "The man who has sinned is the man who must die" and "A son is not to suffer for the sins of his father" (Ez. 18:20)? How can the history so contradict reason?
Therefore, as we look for the true spiritual meaning, seeking to determine whether the events took place typologically, we should be prepared to believe that the lawgiver has taught through the things said. The teaching is this: when through virtue one comes to grips with any evil, he must completely destroy the first beginnings of evil.
For when he slays the beginning, he destroys at the same time what follows after it. The Lord teaches us the same thing in the Gospel, all but explicitly calling on us to kill the firstborn of the Egyptian evils when he commands us to abolish lust and anger and to have no more fear of the stain of adultery or the guilt of murder. Neither of these things would develop of itself, but anger produces murder and lust produces adultery.
Since the producer of evil gives birth to lust before adultery and anger before murder, in destroying the firstborn he certainly kills along with it the offspring which follows. Take for an example a snake: when one crushes his head, he kills the rest of the body at the same time.
This would not have happened unless the blood which turns aside the destroyer had been poured out on our doors. And if it is necessary to perceive the meaning presented here more fully, the history provides this perception in both the killing of the firstborn and the safeguarding of the entrance by blood. In the one the first impulse to evil is destroyed, and in the other the first entrance of evil into us is turned away by the true Lamb. For when the destroyer has come inside, we do not drive him out by our own devices, but by the Law we throw up a defence to keep him from gaining a foothold among us.
Safety and security consists in marking the upper doorpost and the side posts of the entrance with the blood of the lamb. While in this way Scripture gives us through figures a scientific understanding of the nature of the soul, profane learning also places it before the mind, dividing the soul into the rational, the appetitive, and the spirited. Of these parts we are told that the spirit and the appetite are placed below, supporting on each side the intellectual part of the soul, while the rational aspect is joined to both so as to keep them together and to be held up by them, being trained for courage by the spirit and elevated to the participation in the Good by the appetite.
As long, therefore, as the soul is kept safe in this manner, maintaining its firmness by virtuous thoughts as if by bolts, all the parts cooperate with one another for good. The rational for its part furnishes safety to its supports and in its turn receives from them an equal benefit.
But if this arrangement should be upset and the upper become the lower - so that if the rational falls from above, the appetitive and spirited disposition makes it the part trampled upon - then the destroyer slips inside. No opposition from the blood resists his entrance; that is to say, faith in Christ does not ally itself with those of such a disposition.
For he says first to anoint the upper doorpost with blood, then to touch both side doorposts in the same way. How therefore would one anoint the upper first unless it be found on top?
Do not be surprised at all if both things - the death of the firstborn and the pouring out of the blood - did not happen to the Israelites and on that account reject the contemplation which we have proposed concerning the destruction of evil as if it were a fabrication without any truth. For now in the difference of the names, Israelite and Egyptian, we perceive the difference between virtue and evil. Since the spiritual meaning proposes that we perceive the Israelite as virtuous, we should not reasonably require the firstfruits of virtue's offspring to be destroyed, but rather those whose destruction is more advantageous than their cultivation.
Consequently we have been taught by God that we must destroy the firstfruits of the Egyptian children so that evil, in being destroyed as its beginning, might come to an end. And this insight agrees with the history, for the protection of the Israelite children took place through the pouring out of blood in order that good might come to maturity. But what would come to maturity in the Egyptian people was destroyed before it matured in evil.
Life of Moses (Book 2), New York: Harper Collins, 2006, p.56-59.