The writer of Hebrews is inescapably clear about the singular nature of Christ’s sacrifice.
For Christ did not enter a holy place made with hands, a mere copy of the true one, but into heaven itself, now to appear in the presence of God for us; nor was it that He would offer Himself often, as the high priest enters the holy place year by year with blood that is not his own. Otherwise, He would have needed to suffer often since the foundation of the world; but now once at the consummation of the ages He has been manifested to put away sin by the sacrifice of Himself. And inasmuch as it is appointed for men to die once and after this comes judgment, so Christ also, having been offered once to bear the sins of many, will appear a second time for salvation without reference to sin, to those who eagerly await Him. (Hebrews 9:24-28, emphasis added)
Scripture does not waver on the finality of Christ’s sacrifice on our behalf. He came to make a one-time offering for sin, never to be repeated. It was a contrast to the Mosaic covenant, which required a system of near-constant sacrifices. But none of the Old Testament sacrifices could actually atone for sin. They could only serve as a reminder of God’s deliverance and foreshadow Christ’s final sacrifice which would conquer sin.
In the practice of the mass, the Roman Catholic Church has reinstituted an unbiblical system of repeated sacrifices, blaspheming Christ and perverting His work on the cross.
How important is the mass to Catholicism? The Catechism of the Catholic Church refers to it as “the source and summit of the Christian life.” That is to say, it is the origin and the high point of the Catholic faith.  Catechism of the Catholic Church, Paragraph 1324. It’s not peripheral—it’s the heart and soul of the entire system.
In his book The Faith of Millions, John O’Brien, a Catholic priest, explains the procedure of the mass.
When the priest pronounces the tremendous words of consecration, he reaches up into the heavens, brings Christ down from His throne, and places Him upon our altar to be offered up again as the Victim for the sins of man. It is a power greater than that of monarchs and emperors: it is greater than that of saints and angels, greater than that of Seraphim and Cherubim. Indeed it is greater even than the power of the Virgin Mary. While the Blessed Virgin was the human agency by which Christ became incarnate a single time, the priest brings Christ down from heaven, and renders Him present on our altar as the eternal Victim for the sins of man—not once but a thousand times! The priest speaks and lo! Christ, the eternal and omnipotent God, bows His head in humble obedience to the priest’s command.  Rev. John A. O’Brien, The Faith of Millions, revised ed. (Huntington, IN: Our Sunday Visitor, Inc., 1974) 255–56.
Put simply, the Catholic Church won’t let Christ off the cross. In the mass, the substance of the bread and the wine are supposedly transformed into the actual body and blood of Jesus, rendering Him as a repeated, incomplete sacrifice for sins. He’s not Lord and Savior—He’s the eternal Victim, perpetually bound to the altar by the power of the priest, visibly and ubiquitously symbolized in the Roman Catholic crucifix.
That’s a direct denial of Paul’s teaching in Romans 6:8-10.
Now if we have died with Christ, we believe that we shall also live with Him, knowing that Christ, having been raised from the dead, is never to die again; death no longer is master over Him. For the death that He died, He died to sin once for all; but the life that He lives, He lives to God. (emphasis added)
By denying the singular sacrifice of Christ, Catholicism imbues its illegitimate priesthood with artificial power and authority, enslaving its followers to a repetitious system of ineffective, ungodly offerings for sin. It’s essentially paganism sprinkled with enough Christian terminology to deceive and delude souls, convincing them Christ’s death on the cross was not enough to accomplish their salvation. In effect, the mass cancels out the real meaning of the cross.
In Light from Old Times, J. C. Ryle explained the theological and spiritual implications—and imperfections—of the Catholic mass.
Whatever men please to think or say, the Romish doctrine of the real presence, if pursued to its legitimate consequences, obscures every leading doctrine of the gospel, and damages and interferes with the whole system of Christ’s truth. Grant for a moment that the Lord’s Supper is a sacrifice, and not a sacrament—grant that every time the words of the consecration are used the natural body and blood of Christ are present on the communion table under the forms of bread and wine—grant that every one who eats that consecrated bread and drinks that consecrated wine does really eat and drink the natural body and blood of Christ—grant for a moment these things, and then see what momentous consequences result from these premises. You spoil the blessed doctrine of Christ’s finished work when He died on the cross. A sacrifice that needs to be repeated is not a perfect and complete thing. You spoil the priestly office of Christ. If there are priests that can offer an acceptable sacrifice to God besides Him, the great High Priest is robbed of His glory. You spoil the scriptural doctrine of the Christian ministry. You exalt sinful men into the position of mediators between God and man. You give to the sacramental elements of bread and wine an honour and veneration they were never meant to receive, and produce an idolatry to be abhorred of faithful Christians. Last, but not least, you overthrow the true doctrine of Christ’s human nature. If the body born of the Virgin Mary can be in more places than one at the same time, it is not a body like our own, and Jesus was not “the second Adam” in the truth of our nature.  John Charles Ryle, Light from Old Times, 2nd ed. (London, UK: Wycliffe House, 1898), 45.
In simple terms, the mass has nothing to do with the Christian gospel, nothing to do with the Christian life, and nothing to do with the Christian church. It rejects the true, biblical nature of God, Christ, sin, salvation, atonement, and forgiveness. It robs the cross of its meaning and replaces it with superficial, man-centered idolatry. It’s a lie, a fraud, and a damning fabrication that enslaves hearts and ushers people into hell.