The priestly ministry of Christ is one in which He need not make any further sacrifices. His work has been brought about to the fullest extent possible. The Book of Hebrews presents His Atonement as being accomplished one time forever, without further sacrifices. For the author of Hebrews, that which is unchangeable entails perfection. That is the very nature of Christ's position as High Priest. No one else can legitimately lay claim to it.

          The description of His atonement being made "once for all" emphasizes its perfection. It easily contrasts with the animal sacrifices performed under the Levitical priesthood. Those needed to be done on a daily basis. Those needed to be done on an annual basis. There was simply no end to them. This is not the case with Christ's work on the cross. His work had such an effect that animal victims are no longer necessary to be given for offerings. 

          The Penny Catechism has this excerpt explaining the sacrifice of the Roman Catholic Mass as being, "...one and the same Sacrifice with that of the Cross, inasmuch as Christ, who offered himself, a bleeding victim, on the Cross to his heavenly Father, continues to offer himself in an unbloody manner on the altar, through the ministry of his priests."

          If the sacrifice of Jesus Christ at Calvary and the sacrifice of the Mass are one and the same event, then why does it have to be continually offered? If His atonement is ongoing, then it is also not finished. The Mass causes the atonement of Jesus Christ to be just like the repetitive Old Testament sacrifices, which cannot bring about the perfection of our souls. The Mass is not consistent with the presentation of His work in Hebrews as to why it is superior to the animal sacrifices.