http://www.liberius.net/livres/Le_livre_des_figures_prophetiques_000000157.pdf
That web site has a great digitized format of this big book. Copy/paste simple text then translate in you favorite app. A real TREASURE for the Old and New Testaments
AI translation, and corrected!.
From an old Catholic French Book: " Le Livre des Figures Prophétiques ou l'Histoire de Jésus-Christ, de la Ste Vierge, des Apotres..." 487 pages c. 1885 AD.
pp. 198 - 202

Chapter XIX
THE BRAZEN SERPENT, a figure of the Cross of Jesus Christ and its effects.

  The people of Israel, unruly, tired of the hardships of the desert, murmured against God and Moses: "Why have you taken us out of the land of Egypt to make us die in the desert? There is neither bread nor water; our soul is disgusted with this miserable bread." Thus they spoke of the manna. The Lord, to punish them, sent poisonous serpents whose bite was burning. The Israelites confessed their sin to Moses and begged him to intercede for them. Moses prayed for the people, and the Lord, being moved, said to him: 'Make yourself a bronze serpent, and raise it up on a piece of wood as a sign', (so that it may be seen from all sides);  'and all those who are wounded' (by serpents) 'and all who look at it,' (will be healed by this sight and) will live. Fac serpentem æneum, et pone, eum pro signo (Hebr. Super malum, lignum vexillare); qui percussus aspexerit eum, vivet. (Num. XXI:6-9). 'Moses therefore made a bronze serpent, and raised it on a piece of wood as a Sign': This wood had the shape of a mast with its yardarm or of a cross on which a bronze serpent had been fastened. 'Then whoever was wounded by a serpent, looked at the bronze serpent, and he lived'.

'He who looked at this serpent, was healed, not by this serpent which he saw', says the Scripture, Wisdom. XVI. 7, 'but by You Yourself, Lord, who are the Savior of all men,' and their future Messiah.

Jesus Christ Himself explained this figure to us.  He taught us that it marked His future elevation on the cross: All those who are wounded to death by sin, which entered the world as a serpent, whether by original sin or by actual sin, will be healed and regenerated to life through faith in the Son of God lifted up on the cross:
  'As Moses lifted up the Serpent in the wilderness, even so must the Son of Man be lifted up, that whoever believes in him should not perish but have eternal life.' (St. John III:14-15).
***
Notes comparing the Old Testament:
The bites of fiery serpents were the plague that God sent against the Hebrews as punishment for their crimes. A cruel death was the punishment for sin.
The Brazen Serpent resembled the evil serpents, except for their venom, which it did not have.

Comparing now with New Testament:
Subjection to demons, which are real flaming serpents, the penalty of eternal fire and eternal death, were to be the reward for human sin. Sins are also themselves serpents that poison human beings and cause them to die by burning wounds.
Jesus-Christ took on the exterior of flesh similar to that which is criminal, but without taking its venom. "He had flesh like the flesh of sin, but He was without sin" (Heb. IV:15) and He was a Victim for sin.

Comparing Old Testament:
The Serpent, erected by Moses, as a Sign, is raised in the air in the sight of all the people, so that the Hebrews may see it, remember their sins and the wounds caused by their sins, that they may then have recourse to the mercy of God and thus be healed.

New Testament:
  Jesus Christ, who also was 'a sign' (St. Luke, II, 14), 'a signal for the peoples' (Is. XI, 10), was likewise raised in the air and suspended on the wood; from all sides one can see Him on this raised wood, which He chose to show Himself to the whole earth. The eyes of the faithful, wounded by sin, turn to Him as to the author and consomation of their faith; seeing Him they are healed. By believing in Him they are saved.  By trusting in the merit of His death, they are delivered from eternal death.
********
— Here is the remarkable doctrine of St. Justin (1) and of all Christian antiquity on this fact:

  "The signs represented by Moses offer you, O J*ws, the means of recognizing Him who is Christ... For, tell me, is it not God, who, by the mouth of Moses, forbade to make any image or figure of anything that is in Heaven and on earth? Why then did this same God, in the desert, order Moses to raise a bronze serpent, and to represent it by a sign that healed the bites of serpents? Will you accuse God of contradicting Himself?

  Do you not see that He announced by this sign the great mystery of the Cross, which was to destroy the power of the Serpent whose cunning had, through Adam, introduced sin into the world;  that He wanted to teach those who believed in Him who was to suffer by this Sign, that is, by the cross, that He was truly their salvation and the only One who could heal them from all the bites of the Serpent; and by these bites He meant all evil actions, all injustice, all acts of idolatry. And if this is not how you understand it, tell me why Moses had this Serpent raised in the form of a cross? why did he enjoin all those who had been bitten by serpents to look at it (2) in order to be healed, as they were in fact, He who had expressly forbidden to represent the image of any object? Then one of the J*ws who was present applauded this explanation as the only true one. No Hebrew doctor had ever been able to tell him anything satisfactory on this point.

St. Justin replied: "God could certainly have ordered Moses to represent in bronze the image of a serpent, without thereby incurring the reproach of having contradicted Himself: Well! in the same manner, you can find in the Law a sentence of curse against all those crucified, without it striking Christ, the Son of God, through whom God the Father deigns to save all those whose works were worthy of a curse. For you will see that through sin the whole human race is cursed. All are struck with a curse.

  Now, if God the Father wanted his Son to take upon Himself the curses of all men, because He knew well that by delivering Him to death, and to the death of the Cross, He could also call Him back to life, why do you speak (evil) of this Divine Son who resigned Himself to suffering so much in order to obey the will of His Father, as if He had been struck by a curse?

  If Jesus Christ wanted to be represented, not by the Serpent, author and father of sin, but by the serpent attached to the cross and put to death, it was to teach us that He had taken upon Himself sin, and that He destroyed both sin and the author of sin, by attaching them to the instrument of death. If Jesus Christ thus made Himself a curse, it was to destroy the curse that weighed on the human race.

  Such is the salutary effect of the crucifixion of Jesus Christ; such is the admirable teaching of the 'prophetic figure of the Brazen Serpent.' — Wounded to death by the sin which entered the world through the Serpent, let us give thanks to God that we are healed, resurrected to life by faith in the Son of God, by the justice of the Just One par excellence, suffering for us on the cross, as if He had been the Sinner par excellence!

- - - -
*1 Dialogue of St. Justin with the j*wTryphon, nos. 94 and 112.
*2 The healing did not come from the Serpent, but from Christ, represented by the crucified Serpent. This is what the Sage says in these words: 'Etenim neque herba, neque malagna sanavit eos; Sed tuus, Domine, Sermo, qui sanat omnia.' (Wisdom XVI, 12). From then on, it was Jesus Christ, the Word of God, who miraculously healed — Jesus Christ, who had taken the form of the author of sin, to expiate sin by Himself, and to heal the mortal wounds in those who were afflicted by it.

The End. May 4, 2025 AD.