In Genesis 14 we find our first mention of Melchizedek (aka Melchisedec). When Abraham returned from war, Melchizedek came to meet him.

Here's what it says of Melchizedek in that passage:

Genesis 14:18-20 And Melchizedek king of Salem brought forth bread and wine: and he was the priest of the most high God. And he blessed him, and said, Blessed be Abram of the most high God, possessor of heaven and earth: And blessed be the most high God, which hath delivered thine enemies into thy hand. And he gave him tithes of all.

Who was this man? He's not mentioned again in the Old Testament except for David's reference to him in Psalm 110:4, "The LORD hath sworn, and will not repent, Thou art a priest for ever after the order of Melchizedek." That's significant inasmuch as Psalm 110 is so often quoted in the New Testament as a Messianic Psalm. The writer of Hebrews (Paul, I believe) builds upon these two Old Testament references and develops the doctrine of the priesthood of Jesus after the order of Melchizedek.

Paul begins his explanation in Hebrews 5:6 when he quotes Psalm 110:4 in reference to Jesus Christ. Chapters 5-7 develop this priesthood; three verses, Hebrews 7:1-3 are particularly significant:

For this Melchisedec, king of Salem, priest of the most high God, who met Abraham returning from the slaughter of the kings, and blessed him; To whom also Abraham gave a tenth part of all; first being by interpretation King of righteousness, and after that also King of Salem, which is, King of peace; Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like unto the Son of God; abideth a priest continually.

Was he a mere man? From the description given, I'm quite certain that Paul intends for his readers to see in Melchizedek the attributes of God himself. Therefore, I'm convinced that Melchizedek was an incarnation of the Godhead appearing before Abraham. Melchizedek becomes the cornerstone of the high priesthood of Jesus Christ and thus the foundation for our own priesthood as believers. To put it into perspective, Melchizedek is the permanent, supernatural high priest for the real tabernacle in Heaven; Aaron and his descendants were the temporal priests for the pattern of the real tabernacle on earth. The priesthood of Melchizedek therefore supercedes the priesthood of Aaron. In other words, the priesthood of Melchizedek never ceased; the priesthood of Aaron was earthly (not heavenly) and existed as a reflection on earth of the true heavenly priesthood for the period of time from the wilderness to the crucifixion of Christ.