Many dying in the wilderness from the attack of the serpents, Moses, by commandment of the Lord, lifted up a brazen serpent: and those who looked upon it were immediately healed. The lifting up of the serpent is the death of Christ; the cause, by a certain mode of construction, being put for the effect. The serpent was the cause of death, inasmuch as he persuaded man into that sin, by which he merited death. Our Lord, however, did not transfer sin, i. e. the poison of the serpent, to his flesh, but death; in order that in the likeness of sinful flesh, there might be punishment without sin, by virtue of which sinful flesh might be delivered both from punishment and from sin.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Pecc. mer. et remiss. c. xxxii