There have been many speculations as to who exactly this Melchizedek was. The speculations range from the possible to the absurd. Here is a list of some of those speculations.
He was the pre-incarnate Christ. This is a popular notion.
He was the Holy Spirit.
He was an angel.
He was Enoch. By the time Abraham meets Melchizedek, Enoch had been gone for more than a thousand years.
He was Shem, the son of Noah.
He was an extra-ordinary emanation of deity.
The only one of these speculation that bears any kind of merit is that he may have possibly been Shem the son of Noah. This is physically possible for Shem and Abraham were contemporaries. In fact, Shem did not die until after Isaac married. As far as any of the rest of the speculation as to the manner of being Melchizedek was, the Hebrew writer leaves no room for speculation. He was a man.
Melchizedek in not a proper name. It is a title. The ancient kings of pre-Israel Jerusalem were called the Tsedeks. Melchizedek is from Meleck meaning King and Tsedek meaning righteousness. Thus, king of righteousness. He was the King of Salem meaning peace. This Salem would later be called Jerusalem meaning foundation of peace. In Joshua 10:1 we encounter another Tsedek of Salem called Adoni-Tsedek meaning lord of righteousness. The difference between these to men in the deterioration of the worship from the time of Melch-Tsedek to Adoni-Tsedek.
The nature of Melchizedek - He was a man.
"Now consider how great this man was... ."
The word "man" is not represented in the text by either ἄνθρωπος nor ἀνήρ. It is provided by the gender of the pronoun οὗτος which is nominative masculine singular for "this one," thus, "this man."
The fact that he was a High Priest of God demands that he was of the human race. In 5:1 we learn that every High Priest is taken from among men.
As a man, he had a genealogy. "Whose genealogy was not derived from them" (the Levites). This is in the possessive which declares that he had a genealogy but, that his genealogy was not traced from the priestly tribe of Levi.
He was a high priest without genealogy.
"Without father, mother, or genealogy." Like Jesus, Melchizedek does not receive his priesthood from his a predecessor. In the Levitical system, the high priest was descended only through the line of Aaron. 1Chronicles 6:50-52. But, the office of the high priest was not passed on to Melchizedek by his father, nor did he in turn pass it on to his heir. In other words, his is a one-man-forever-priesthood.
"Having neither beginning of days nor end of life."
In this there are three possibilities.
That this refers to the person of Melchizedek the man. Some argue from this that Melchizedek was not a man but some supernatural being who was neither born of human parents not had a beginning or end of life. But, as the text says, he was a man and as such, he had a past, verse 6. Some view this with the preceding statement as simply a Hebraism which stresses the obscurity of his genealogy and posterity. Perhaps.
That this refers not to the man himself but to his priesthood. This priesthood is unlike that of the Levitical system. We can look back at Sinai and see where the Levitical priesthood had its beginning of days with the anointing of Aaron and his sons, Exodus 28:1ff. We can then look forward from there to the cross and see where this priesthood saw its end of life. Now, a new and greater covenant is inaugurated in Jesus "according to the power of an endless life." But, this may not apply to just the priesthood apart from the man because this is a one man priesthood and apart from the man there is no priesthood.
That this refers to the man as a high priest. As a man he had a beginning of days and an end of life. As high priest, he has neither but remains a priest continually. This contrasts the priests of the Levitical system whose "beginning of days" began at the age of twenty-five when they began to serve as priests. They reached their "end of life" at the age of fifty when they completed their appointed time of priestly service, Numbers 8:24-25.
"But made like the Son of God."
Here, the order is reversed. In 6:20, Christ is presented as a High Priest after the order of Melchizedek. Now, Melchizedek is said to be a High Priest who was made like the Son of God. Like everything else that is type, Melchizedek is the shadow of the reality. This is like the building of the tabernacle in Exodus 25:40 being built according to the "pattern shown to you on the mountain." Everything that is shadow must be patterned according to the substance it represents. The substance ALWAYS precedes the type. It must reflect the reality.
He "remains a priest continually." His priesthood is uninterrupted even by death. He leaves his office to no one else. Although Melchizedek has been dead for many centuries, he is still the central figure in the one man forever priesthood. Like the Son of God, he carries his priest beyond the grave. His priesthood, in contrast to that of the Levites is not bound by the physical - "not according to the law of fleshly commandment," 15-16. This fleshly commandment says that the Levitical priest must end his days of service at the age of 50. The High Priest ended his days of service at his death. In contrast, the priesthood of Melchizedek is greater. He continues as the High Priest of his priesthood even though he is dead, 8.