-- Letter to the Hebrews, 7:25—8:6
Jesus is always able to save those who approach God through him, since he lives forever to make intercession for them.
It was fitting that we should have such a high priest: holy, innocent, undefiled, separated from sinners, higher than the heavens. He has no need, as did the high priests, to offer sacrifice day after day, first for his own sins and then for those of the people; he did that once for all when he offered himself. For the law appoints men subject to weakness to be high priests, but the word of the oath, which was taken after the law, appoints a son, who has been made perfect forever.
The main point of what has been said is this: we have such a high priest, who has taken his seat at the right hand of the throne of the Majesty in heaven, a minister of the sanctuary and of the true tabernacle that the Lord, not man, set up. Now every high priest is appointed to offer gifts and sacrifices; thus the necessity for this one also to have something to offer. If then he were on earth, he would not be a priest, since there are those who offer gifts according to the law. They worship in a copy and shadow of the heavenly sanctuary, as Moses was warned when he was about to erect the tabernacle. For God says, “See that you make everything according to the pattern shown you on the mountain.” Now he has obtained so much more excellent a ministry as he is mediator of a better covenant, enacted on better promises.
Christ's eternal loving sacrifice of all of Himself to His Father, through the love of the Holy Spirit, takes place forever in Heaven, where it is the summit of the ever-joyful, ecstatically self-sacrificing love of the Three Persons of the Trinity for Each Other and for us. Christ's sacrificial death on the Cross happened only once in the earthly flow of time, but the eternity of God in Heaven is forever in contact with that one moment in ancient Jerusalem, when the veil of the old Temple was torn by an earthquake at the moment of Christ's death, and when His Body, the new Temple, was destroyed by the iron nails of our sins shedding His Blood on the wood of the Cross, new Tree of Life, only for His Bodily Temple to be rebuilt, resurrected three days later.
When Protestant complain that the Sacrifice on the Cross was offered once and for all on this earth in ancient Jerusalem, and thus that we ought not to try to repeat that sacrifice at our Sacrifice of the Mass, they misunderstand. This is unsurprising--it was the Catholic Church that canonized the Bible, and we might be expected to understand our own Book better than those who refuse to listen to the Church founded by Christ's Apostles upon the rock of Peter. The Sacrifice of the Mass isn't a do-over of the Crucifixion. It is a participation, beyond space-time, in the Cruxifixion.
When we partake of the Sacrament of His Body in Blood in the Eucharist at the Sacrifice of the Mass, we are eternally connected to Christ's joyful, lovingly sacrificial priesthood before the Father in Heaven, where God's angel brings the matter of our humble offering of bread and wine up to the Father as the Holy Spirit replaces it with Christ's Body and Blood.
When eat this Bread and drink this Blood, we are taken up across spacetime into the super-organism of the Church, the Mystical Body of Christ the Vine throughout the space of the Earth and the time of the centuries. The Flesh we eat and Blood we drink is not a repetition of the Sacrifice on the Cross. The Flesh and Blood on our altars, in every little tiny parish Church, no matter how tacky the architecture or slummy the neighborhood, is exactly the flesh of His Sacred Heart beating its last in ancient Jerusalem, exactly the Blood pouring from His battered hands and feet. It is 2,000 years away right now while you hold it.
The altar is a time machine. Under the appearance of Bread, the Host you take and eat is actually, right now, in your mouth in this century, and yet also in ancient Jerusalem, beating in His Sacred Heart. That is how the Vine reaches across spacetime to engraft us into His Body. If you are ever bored at Mass, remember that Jesus Christ, Who once forged the furnace of the Sun and sculpted the mighty ice-rings of Saturn more effortlessly than an ancient Jewish carpenter in a humble workshop might hew a table, is really in the room with you right now, yet also on the Cross in ancient Judea, yet also eternally (outside spacetime past or present, ancient or modern) rejoicing at His altar before the throne of the Father in Heaven. You are at the foot of the Cross and in the throne room of Your Maker and the Maker of the Cosmos right now. You are participating in the Life of three places at once, the Cross, Heaven, and here, and only one of them is this dinky little parish church. If you were visiting the Oval Office in the White House, you'd be excited: this is a way bigger deal.