The Son of Man must be lifted up

The Sign of the Brazen Serpent
Wherefore the Lord sent among the people fiery serpents, which bit them and killed many of them. Upon which they came to Moses, and said: We have sinned, because we have spoken against the Lord and thee: pray that he may take away these serpents from us. And Moses prayed for the people. And the Lord said to him: Make brazen serpent, and set it up for a sign: whosoever being struck shall look on it, shall live. Moses therefore made a brazen serpent, and set it up for a sign: which when they that were bitten looked upon, they were healed. (Numbers 21:6-9)
The Word of the Cross
For the word of the cross, to them indeed that perish, is foolishness; but to them that are saved, that is, to us, it is the power of God. (1 Corinthians 1:18)
I Will Draw All Things to Myself
Now is the judgment of the world: now shall the prince of this world be cast out. And I, if I be lifted up from the earth, will draw all things to myself. (Now this he said, signifying what death he should die.) The multitude answered him: We have heard out of the law, that Christ abideth for ever; and how sayest thou: The Son of man must be lifted up? Who is this Son of man? Jesus therefore said to them: Yet a little while, the light is among you. Walk whilst you have the light, that the darkness overtake you not. And he that walketh in darkness, knoweth not whither he goeth. Whilst you have the light, believe in the light, that you may be the children of light. (John 12:31-36)
The Holy Sacrifice of the Mass
The Holy Sacrifice of the Mass is the mystery of the Cross at the heart of the Church. It is the efficacious enactment of what Saint Paul calls the verbum Crucis, the word/event of the Cross. The unbloody Sacrifice of the Mass, offered from the rising of the sun to its setting, and that bloody Sacrifice offered once upon the Cross are one and the same sacrifice. In Mediator Dei, the Venerable Pope Pius XII, referring to Session 22 of the Council of Trent, affirms that,

The august sacrifice of the altar, then, is no mere empty commemoration of the passion and death of Jesus Christ, but a true and proper act of sacrifice, whereby the High Priest by an unbloody immolation offers Himself a most acceptable victim to the Eternal Father, as He did upon the cross. “It is one and the same victim; the same person now offers it by the ministry of His priests, who then offered Himself on the cross, the manner of offering alone being different.”

Behold the Lamb of God
Once the celebration of the Holy Sacrifice is ended, the adorable Body of Christ, the saving flesh of the Lamb, the pure victim, the holy victim, the immaculate victim, is reserved in the tabernacle for the Holy Communion of those close to death, of the sick, of prisoners, and of the homebound. The Lamb of God, being truly present in the Most Holy Sacrament of the Altar, remains, at every hour of the day and night, worthy of adoration, praise, love, and thanksgiving.
Christus Passus
I shall never forget the passionate conviction with which my old professor, Father Thomas Urban Mullaney O.P. taught the mystery of Christus passus in his course on the Most Holy Eucharist. It is something that marked me profoundly thirty-five years ago, and that continues to affect my life and shape my piety. Jesus Christ in the Most Holy Sacrament of the Altar remains in the very state and dispositions that animated Him in the supreme hour of His Sacrifice on the Cross. The Most Holy Eucharist is the presence of Christus passus, that is, Christ in the very act of His self-offering to the Father.
The Mystic Reality of Every Mass
The exaltation of the Holy Cross is not only the lifting up of the saving wood soaked in the Most Precious Blood; it is also the lifting up of the Lamb immolated upon it as upon an altar. This is the mystic reality of every Holy Mass. That same mystic reality is prolonged in the adoration of the Lamb, living and present in the Most Holy Sacrament.
Set It Up for a Sign
When the adorable Body of Christ is withdrawn from the tabernacle and exposed to the gaze of the faithful in the monstrance, the word of the Lord to Moses is wondrously and perfectly fulfilled: “And the Lord said to him: Make brazen serpent, and set it up for a sign: whosoever being struck shall look on it, shall live. Moses therefore made a brazen serpent, and set it up for a sign: which when they that were bitten looked upon, they were healed.” (Numbers 21:6-9)
Look Upon the Body of Christ
Look, then, upon the Body of Christ, and see the sign of the brazen serpent fulfilled and surpassed in a manner that only faith can grasp. Look upon the Body of Christ, and see the source of all healing, the remedy for souls poisoned by the bites of the fiery serpents of the world, the flesh, and the devil. Look upon the Body of Christ and be drawn into the pierced Heart of the Lamb.
I Will Draw All Men to Myself
For souls called to a life of Eucharistic adoration, the exaltation of the Holy Cross — in the fullest meaning of the phrase — is the reality of every day and of every hour. “Yes,” says the Lord, “if only I am lifted up, I will draw all men to myself.” (John 12:32)