We have written before on how the Old Covenant was fulfilled in the New Covenant. The writer of Hebrews, writing to Jews (Hebrews) thoroughly examines how this was so. We found the fulfillment of the New Covenant in Worship in the Mass of the Catholic Church.
If perfection had been achieved through the levitical priesthood (on the basis of which the people received the law), what need would there have been to appoint a priest according to the order of Melchizedek, instead of choosing a priest according to the order of Aaron?
When there is a change of priesthood, there is necessarily a change of law. Now he of whom these things are said was of a different tribe, none of whose members ever officiated at the altar. It is clear that our Lord rose from the tribe of Judah, regarding which Moses said nothing about priests.
The matter is clearer still if another priest is appointed according to the likeness of Melchizedek: one who has become a priest, not in virtue of a law expressed in a commandment concerning physical descent, but in virtue of the power of a life which cannot be destroyed. Scripture testifies: "You are a priest forever according to the order of Melchizedek."
The former commandment has been annulled because of its weakness and uselessness, for the law brought nothing to perfection. But a better hope has supervened, and through it we draw near to God.
This has been confirmed by an oath. the priests of the old covenant became priests without an oath, unlike Jesus to whom God said:
"The Lord has sworn, and he will not repent:
'You are a priest forever, according to the order of Melchizedek.'"
Thus has Jesus become the guarantee of a better covenant.
Under the old covenant there were many priests because they were prevented by death from remaining in office; but Jesus, because he remains forever, has a priesthood which does not pass away. Therefore he is always able to save those who approach God through him, since he forever lives 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. Unlike the other high priests, he has no need 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 sets up as high priests men who are weak, but the word of the oath which came after the law appoints as priest the Son, made perfect forever. --Hebrews 7:11-28
You have learned through your catechesis what The Church teaches on the Priesthood of Christ. You may want to revisit that section of the Catechism to refresh your gratitude for the worship that takes place during the Mass. You may want to revisit that section to remember why we as Catholic Christians submit to the Sacred Authority of The Church. You may remember what we have written on how the Sacred Authority of The Catholic Church drew our hearts and minds (faith and reason) to the sure foundation of The Faith. (See our July 8 and 31, 2015 posts) To engage your Protestant friend in appreciation of the Sacred Authority of The Church, you may want to refer them to these links from the Catechism before you tell them of your own gratitude for the stability of The Catholic Church.
http://www.vatican.va/archive/ENG0015/__PE.HTM through http://www.vatican.va/archive/ENG0015/__PT.HTM
http://www.vatican.va/archive/ENG0015/__P6S.HTM through http://www.vatican.va/archive/ENG0015/__P77.HTM
The early Church Fathers' writings are often cited in the Catechism and there are many you may want to refer to while you read through the sections listed above. We include another writing we were reminded of in today's readings of the Divine Office. Below is a treatise on faith by Saint Fulgentius of Ruspe. St. Fulgentius was named bishop of Ruspe, Tunisia, Northern Africa in 508 A.D. He was friend to St. Augustine of Hippo, and he wrote eloquent defenses of orthodox Catholic doctrines.
The sacrifices of animal victims which our forefathers were commanded to offer to God by the holy Trinity itself, the one God of the old and the new testaments, foreshadowed the most acceptable gift of all. This was the offering which in his compassion the only Son of God would make of himself in his human nature for our sake.
The Apostle teaches that Christ "offered himself for us to God as a fragrant offering and sacrifice." He is the true God and the true high priest who for our sake entered once for all into the holy of holies, taking with him not the blood of bulls and goats but his own blood. This was foreshadowed by the high priest of old when each year he took blood and entered the holy of holies.
Christ is therefore the one who in himself alone embodied all that he knew to be necessary to achieve our redemption. He is at once priest and sacrifice, God and temple. He is the priest through whom we have been reconciled, the sacrifice by which we have been reconciled, the temple in which we have been reconciled, the God with whom we have been reconciled. He alone is priest, sacrifice and temple because he is all these things as God in the form of a servant; but he is not alone as God, for he is with the Father and the Holy Spirit in the form of God.
Hold fast to this and never doubt it: the only-begotten Son, God the Word, becoming man offered himself for us to God as a fragrant offering and sacrifice. In the time of the Old Testament, patriarchs, prophets and priests sacrificed animals in his honor, and in honor of the Father and the Holy Spirit as well. Now in the time of the New Testament the holy Catholic Church throughout the world never ceases to offer the sacrifice of bread and wine, in faith and love, to him and to the Father and the Holy Spirit, with whom he shares one godhead.
Those animal sacrifices foreshadowed the flesh of Christ which he would offer for our sins, though himself without sin, and the blood which he would pour out for forgiveness of our sins. In this sacrifice there is thanksgiving for, and commemoration of, the flesh of Christ that he offered for us, and the blood that the same God poured out for us. On this Saint Paul says in the Acts of the Apostles: "Keep watch over yourselves and over the whole flock, in which the Holy Spirit has appointed you as bishops to rule the Church of God, which he won for himself by his blood."
Those sacrifices of old pointed in sign to what was to be given to us. In this sacrifice we see plainly what has already been given to us. Those sacrifices foretold the death of the Son of God for sinners. In this sacrifice he proclaimed as already slain for sinners, as the Apostle testifies: "Christ died for the wicked at a time when we were still powerless, [and] when we were enemies we were reconciled with God through the death of his Son. --St. Fulgentius of Ruspe