Some Catholic apologists will point to the king and priest of Salem (Jerusalem) in the time of Abram, as proof that the Catholic mass is indeed a sacrifice:

Melchisedech, the king of Salem and a priest, offered sacrifice under the form of bread and wine (Gen 14:18). Psalm 110 [v. 4] predicted Christ would be a priest "according to the order of Melchisedech", that is, offering a sacrifice in bread and wine. We must then, look for some sacrifice other than Calvary, since it was not under the form of bread and wine. The Mass fits the bill.

Source: Catholicism and Fundamentalism, by Karl Keating, copyright 1988 by Ignatius Press, San Francisco, ISBN 0-89870-177-5, bearing the Imprimatur and Nihil Obstat of the Catholic Church, page 253.

Gen 14:17 (KJV) And the king of Sodom went out to meet him after his return from the slaughter of Chedorlaomer, and of the kings that were with him, at the valley of Shaveh, which is the king's dale.
Gen 14:18 And Melchizedek king of Salem brought forth bread and wine: and he was the priest of the most high God.
Gen 14:19 And he blessed him, and said, Blessed be Abram of the most high God, possessor of heaven and earth:
Gen 14:20 And blessed be the most high God, which hath delivered thine enemies into thy hand. And he gave him tithes of all.

Now from the King James Bible, as quoted above, the Catholic claim is simply not apparent. Here is the same verse from the Catholic Douay Rheims Bible as revised in 1899 by Bishop Challoner:

Gen 14:18 But Melchisedech the king of Salem, bringing forth bread and wine, for he was the priest of the most high God.

The essence of Keating's argument is that Melchisedech offered the bread and wine to Abram as a direct result of his role as a priest, as evident by the phrase "for he was the priest", the role of the priest being to offer sacrifices. So by Keating's thinking, Melchisedech offered the "sacrifice" of the Catholic Mass to Abram. Keating dismisses even the possibility that Melchisedech was merely offering Abram a meal of bread and wine in celebration of his recent victory over king Chedorlaomer and his allies, but the text gives no indication of a formal worship ceremony of any kind. Keating, it would seem, is trying to assert that Melchisedech instituted the Lord's Supper with Abram, rather than Jesus with His disciples. Note the following similar incident that involved David and the priest Ahimelech:

1 Sam 21:2 (NIV) And David said unto Ahimelech the priest, The king hath commanded me a business, and hath said unto me, Let no man know any thing of the business whereabout I send thee, and what I have commanded thee: and I have appointed my servants to such and such a place.
1 Sam 21:3 Now therefore what is under thine hand? give me five loaves of bread in mine hand, or what there is present.
1 Sam 21:4 And the priest answered David, and said, There is no common bread under mine hand, but there is hallowed bread; if the young men have kept themselves at least from women.
1 Sam 21:5 And David answered the priest, and said unto him, Of a truth women have been kept from us about these three days, since I came out, and the vessels of the young men are holy, and the bread is in a manner common, yea, though it were sanctified this day in the vessel.
1 Sam 21:6 So the priest gave him hallowed bread: for there was no bread there but the showbread, that was taken from before the LORD, to put hot bread in the day when it was taken away.

Here David is actually given the consecrated Showbread by the priest, not because David wanted to have a communion service, nor was it the intent of the priest to conduct a "sacrificial" ceremony for David, but rather the bread was given because David and his men were hungry and it was the only food available.

Mark 2:25 (NIV) He answered, "Have you never read what David did when he and his companions were hungry and in need?
Mark 2:26 In the days of Abiathar the high priest, he entered the house of God and ate the consecrated bread, which is lawful only for priests to eat. And he also gave some to his companions."

There is no reason to believe that Melchisedech was doing anything else than simply offering Abram food and drink, similar to that as happened with David and his men.

As to Psalm 110:

Psa 110:1 (KJV) A Psalm of David. The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.
Psa 110:2 The LORD shall send the rod of thy strength out of Zion: rule thou in the midst of thine enemies.
Psa 110:3 Thy people shall be willing in the day of thy power, in the beauties of holiness from the womb of the morning: thou hast the dew of thy youth.
Psa 110:4 The LORD hath sworn, and will not repent, Thou art a priest for ever after the order of Melchizedek.
Psa 110:5 The Lord at thy right hand shall strike through kings in the day of his wrath.

Keating states:

What is more, "according to the order of Melchisedech" ("order" not referring, of course, to the modern notion of a religious order, there being no such thing in Old Testament days.) The only "manner" shown by Melchisedech was the use of bread and wine. A priest sacrifices the items offered - that is the main task of all priests, in all cultures, at all times - so the bread and wine must have been what Melchisedech sacrificed. He did not bring these elements along just because he thought it might be time for Abraham's lunch. [pages 253-254]

What Keating overlooks here is that Melchisedech was both a priest and the king of Salem (Jerusalem), but he was not of the Levitical priesthood (Levi being a descendant of Abraham). This is the "order of Melchisedech" scripture refers to so frequently (See Hebrews 5:6, 10; 6:20; 7:1:3, 11, 15, 17, 24, 28). Jesus is our High Priest, and the King of Kings, yet He was not of the tribe of Levi, but rather the tribe of Judah.

A recent Catholic Bible commentary completely disagrees with Keating:

Recognizing Abraham's great deed - winning a victory that the five kings were not able to achieve - he [Melchizedech] sets a feast before Abraham to win the goodwill of so favored a personage.

Source: The New Jerome Biblical Commentary, published and copyrighted by Prentiss Hall, 1990 and 1968, ISBN 0-13-614934-0, bearing the Imprimatur and Nihil Obstat of the Catholic Church, page 21.

Karl Keating and other Catholic apologists who appeal to Melchisedech to justify the Mass as a "sacrifice" are still a dogma in search of even the slimmest scriptural proof.

http://www.aloha.net/~mikesch/