Reverently opening your copy of the Roman Martyrology you find in the first entry for today
1. Commemoratio sancti Melchisedech, regis Salem et sacerdotis Dei altissimi, qui Abraham benedicens salutavit a victoria regressum, Domino sanctum sacrificium, immaculatam hostiam, offerens, atque in praefiguratione Christi rex pacis et iustitiae interpretatus est et, licet genealogiae expers, sacerdos in aeternum. … The commemoration of Saint Melchisedech, king of Salem and priest of the Most High God, who as he was blessing him greeted Abraham who had returned from victory, offering to the Lord a holy sacrifice, a spotless victim, and who is considered as a foreshadowing of Christ as king of peace and justice, and, even though having no part in a geneology, as a priest forever.
That interpretatus est really must be considered passive, even though usually interpretor is deponent.
Take note of the phrase sanctum sacrificium, immaculatam hostiam which is in the Roman Canon.
We don’t really know too much about his enigmatic figure. He perhaps was King of Jerusalem (Salem) and he was a priest. He encountered Abraham after the defeat of Chodorlahomor recounted in Genesis 14. At that meeting Melchisedech offered bread and wine in thanks to God for the victory. The comment about him not being in any geneology (cf. Hebrews 7) is significant because it gives the impression that Melchisedech had no beginning or end. Together with his titles (king of peace, king of justice, priest of the Most High) this helps give him the mystique of a "type" of Christ.
In a work on the sacraments (De sacramentis) attributed to St. Ambrose of Milan (+397) Melchisedech figures large in the explanation preached to the newly baptized about the meaning of the sacrifice of the Mass. It is an interesting piece for our review in these modern times, when not a small number of Catholics both less grasp the Church’s teaching about the Eucharist or, in many cases, don’t believe. Take note that Ambrose was very concerned in his writings and preaching to demonstrate that Christianity had very ancient roots. In the ancient world, antiquity gave something more authority, rather than less. For example, he worked to show that much of Hellenistic Platonism was inspired by the books of Old Testament. This is just a little intellectual background to this piece which follows.
This long excerpt (not my translation, but my emphasis) is from De sacramentis 4,3ff. Place yourself in one of the ancient churches of Milan in the week after your own baptism deep in the night before Easter day. The bishop is teaching you about the Mass.
The Christian Sacraments are older than the Jewish. You have come to the altar of God, you have seen the sacraments placed there, and you wonder to see there a created thing; nevertheless it is a solemn and an unusual created thing. Someone has said perhaps: God showed great favor to the Jews. He rained manna on them from heaven (Exod. xvi. 15). What more has He given His own faithful; what more has He given to those to whom He promised more?
Take in what I now say. The mysteries of the Christians were before those of the Jews; and more sacred are the sacraments of the Christians than those of the Jews. How can this be? Pay heed to this. Where did the Jews begin? From Judah, the great-grandson of Abraham; or, if you wish to understand it that way, from the Law; that is, when the Jews merited to receive the Law. So they are called Jews from the great-grandson of Abraham, or from the time of the saintly Moses. And if God then rained manna from heaven on the Jews, murmuring against Him, the figure of these holy sacraments preceded this: in Abraham’s time, when he collected three hundred and eighteen well-appointed men, and pursued his enemies, and brought his grandchild back from captivity. Then, returning a victor, there met him Melchisedech the priest, and he offered bread and wine (Gen. xiv. 18).
Who had the bread and wine? Abraham did not have it. But who had it? Melchisedech. He then is the author of the sacraments (Heb. vii. I seq.). Who is Melchisedech? He who is made known to us as the King of Justice, the King of Peace. Who is the King of Justice? Can any man be King of Justice? Who then is King of Justice if not the Justice of God, Who is also the Peace of God, the Wisdom of God? Who could say: Peace I leave with you, my peace I give unto you? (Jn. xiv. 27.)
Let you then grasp, that these sacraments which you receive are prior in time to those of the Law of Moses; whatever the Jews may have to say. And that the Christian people had begun before the Jewish people had begun: we through predestination, they in name.
Melchisedech therefore offered a sacrifice of bread and wine. Who is Melchisedech? He was, says the Apostle, without father, without mother, without genealogy, having neither beginning of days nor end of life, but likened unto the Son of God, continueth a priest forever; and this we read in the Epistle to the Hebrews (vii). Without father, he says, and without mother. In this whom does he resemble? The Son of God. For the Son of God, in His heavenly generation, was born without a mother: He was born of the Father alone. And again when He was born of the Virgin, He was born without father: for He was not begotten of the seed of man, but born of the Holy Ghost (Mt. i. 20) and of the Virgin Mary, and brought forth from her virginal womb, in all things as the Son of God.
Melchisedech was also a priest, as Christ is a priest; to Whom it was said: Thou art a priest for ever according to the order of Melchisedech (Ps. cix. 4). Who therefore is the author of the sacraments if not the Lord Jesus? These sacraments have come down from heaven; from whence all counsel comes. It was a truly great and divine miracle that God should rain manna from heaven on His people; and that the people should eat though they did not work.
But perhaps you will say: My bread is ordinary bread. On the contrary, this bread is bread only before the words of the sacred rite. When the consecration has been added, from being bread it becomes the Body of Christ. Let us therefore prove this. How can that which is bread be the Body of Christ? By consecration. Consecration by what words; by whose words? Those of the Lord Jesus. For all the other words which are said previous to this are said by the priest: the praises that are offered to God, the prayer that is offered for the congregation, for rulers, and for others. But when the moment comes to consecrate the venerable sacrament, the priest will no longer use his own words, but will use the words of Christ. It is therefore the Word of Christ that consecrates this sacrament.
Who is the Word of Christ? Who but He by Whom all things were made. The Lord commanded, and the heavens were made. The Lord commanded, and the earth was made. The Lord commanded and the seas were made. The Lord commanded, and every creature was brought forth (Gen. i). You see then how wondrous in work is the Word of Christ.
If then there is such power in the Word of the Lord Jesus, so that the things that were not by It began to be, how much the more can It change what is into another thing? The heavens did not exist, nor the sea, nor the land, yet hearken to David speaking: He spoke, and they were made. He commanded, and they were created (Ps. cxlviii. 5). And accordingly I answer you; that the bread was not the Body of Christ before the consecration. But I say to you that after the consecration it is now Christ’s Body. He spoke, and It was made. He commanded, and It was created. You were yourself; but you were your old self. After you were consecrated you began to be a new creature. Do you wish to know what sort of new creature? Everyone, says the Scripture, in Christ is a new creature (I Cor. v. 17).
Understand therefore how the words of Christ have changed every creature; and now change, when He wills, the order of nature. You wish to know in what manner? Listen then; and let us take an example from His own birth. It is the rule of nature that a man is born only from the conjugal relation of man and woman. But because the Lord willed it, because He chose this sacred means (sacramentum), Christ, that is, the one Mediator of God and men, the Man Jesus Christ (I. Tim. ii. 5), was born of the Holy Ghost, and the Virgin Mary. You see then how, contrary to the order and custom of nature, a Man was born of a Virgin?
Consider another example. The Jewish people were hemmed in by the Egyptians, and behind them was the sea. By divine command Moses struck the waters with a rod, and the waves divided; not certainly in accord with nature’s laws, but in accord with the grace of the heavenly command (Exod. xiv). And consider another example. The people were thirsty, and they came upon a well. But it was a bitter well. So the saintly Moses cast a certain tree into the well, and the fountain that was bitter was made sweet; that is, it changed the quality of its nature, and was turned into sweetness (Exod xv. 23). Consider a fourth example. An iron axe had fallen into the water, and since it was iron it sank. And Elisaeus cast in a piece of wood, and the iron swam upon the surface of the water; and this purely is contrary to the nature of iron (IV Kgs. vi. 6), which is a far heavier element than water.
From these examples therefore you may understand how great is the power of the heavenly word. If it can work a wonder in an earthly well, if the heavenly word can work wonders in other things, will it not work similarly in the heavenly Sacraments? And so you have learned that the Body of Christ is made from bread; and that wine and water are mingled in the chalice, but that this becomes Blood by the consecration of the heavenly words. But perhaps you will say: ‘I see no appearance of blood.’ But it possesses a likeness to it. For as you have taken on the likeness of his death, so do you also drink the likeness of His Precious Blood: that there may be no horror of spilt blood, and yet that the price of our Redemption may be efficacious. You have therefore learned that what you receive is the Body of Christ.
The words of the Lord make and consecrate His own Body and Blood. And would you know by what heavenly words It is consecrated? Here then are the words. The Priest says: Grant us, he says, that this oblation may be attributed to us, confirmed, an offering of our reason, acceptable to Thee, as the figure of the Body and Blood of our Lord Jesus Christ, Who, on the day before He suffered took bread in His holy hands, looked up to heaven to Thee, Holy Father Almighty, Eternal God, and giving thanks, He blessed, broke it, and gave what was broken to His Apostles and to His Disciples, saying: Take ye, and eat ye all of this; for this is my body, which shall be broken for many (Lk. xxii. 19 V.I.) [FOR "MANY"!!]
In like manner also, on the day before He suffered, after He had supped, He took the chalice, looked up to heaven to Thee, Holy Father Almighty Eternal God, and giving thanks He blessed it and gave to His Apostles and Disciples, saying: Take ye and drink ye all of this; for this is my blood (Mt. xxvi. 27).
Consider all this. There are the words of the Evangelist up to Take ye, whether of the Body or the Blood. From there on they are the words of Christ: Take ye, and drink ye all of this; for this is my blood. Consider each word.
Who, he says, on the day before He suffered took bread in His holy hands. Before it is consecrated it is bread. When the words of consecration have been added, it is the Body of Christ. Then listen to Him saying Take ye, and eat ye all of this; for this is my body. Again, before the words of consecration, it is a chalice filled with wine and water. Where the words of Christ have wrought, there the Blood of Christ, Which has redeemed His people, is made. You see then in how many ways the words of Christ are able to change all things. Lastly, the Lord Jesus Himself testifies to us that we receive His Body and Blood. Are we to doubt His honesty and His testimony?
Now return with me to the main subject of my sermon. It was a great and venerable sign that manna rained from heaven upon the Hebrews (Exod. xvi. 13). But reflect. Which is the greater wonder: the manna from heaven, or the Body of Christ? The Body of Christ, the Creator of heaven. Then again he who ate manna is dead; but he that will eat of This Body his sins will be forgiven him, and he shall not die for ever.
So not without meaning do you say: Amen; in that moment confessing in spirit that you receive the Body of Christ. What the tongue confesses, let the heart hold fast.
That you may know that this is a divine mystery, its Figure preceded it. Learn then how great is this sacrament. Consider what He says: As often as you shall do this, do it in commemoration of Me, until I come again (cf. I Cor. xi. 26). And the Priest says: Mindful therefore of His most glorious passion, and of His Resurrection from the dead, and of His Ascension into heaven, we offer Thee this immaculate Host, this reasoning victim, this unbloody sacrifice, this holy Bread, and the Chalice of eternal life; and we beg and pray that by the hands of the Angels thou wilt receive this Offering upon Thy heavenly altar, as Thou didst deign to receive the gifts of Thy servant, Abel the Just, and the sacrifice of Abraham our father, and that which the High Priest Melchisedech offered to Thee. [Sounds very much like the Roman Canon, no?]
So then, as often as you shall receive, what does the Apostle say to you? As often as you shall receive, you shall announce the death of the Lord. If we announce His death, we announce the forgiveness of sins. If as often as His Blood is shed, it is shed unto the remission of sins, I ought to receive It always; that my sins may always be forgiven. I who am always sinning ought always to have what heals me. [It is thought that Communion was received very often in the Milan of Ambrose, even daily. Also, the preacher is not saying that mere reception of the Eucharist forgives sins. Nevertheless, forgiveness of sins (itself a sacrament) is closely tied to the Eucharist.]
May the Lord our God preserve you in the grace He has given you, and may He deign to enlighten yet more the eyes He has opened, through His only Son, the Lord God our King and Savior, through Whom and with Whom be there to Him, together with the Holy Ghost, praise, honor, glory, magnificence, from all ages, and now, and for ever and ever, world without end. Amen.