There have been many speculations as to who exactly this Melchizedek was. The speculations range for the possible to the absurd. Here is a list of some of those speculations.
1. He was the pre-incarnate Christ. This is a popular notion.
2. He was the Holy Spirit.
3. He was an angel.
4. He was Enoch. By the time Abraham meets Melchizedek, Enoch had been gone for more than a thousand years.
5. He was Shem, the son of Noah.
6. 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 are 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.
This is exactly what my own research has lead me to believe...
If we assume for the moment that when God said to man "
be fruitful & multiply and
have dominion over the fish of the sea, and over the foul of the air, and over every living thing that moves upon the earth", this was God giving Adam the "
crown of earth", as representative of God (i.e. a priest on his earthly throne). We even read that sacrifices were made to God as early as Cain & Abel, so it's plausible
Adam was High Priest & King of Earth. And if this is true, the crown must be passed on after Adam is dead...and likely it was passed on to the next eldest firstborn still alive when each previous King of Earth died.
So if we use Genesis 5 to extrapolate the ages of each firstborn when their father died (warning: this is my own research and has not been confirmed), then we get the following:
Adam dies (age 930) > Seth receives crown (age 801)
Seth dies (age 912) > Enoch receives crown (age 808)
Enoch dies (905) > Kenan receives crown (815)
Kenan dies (910) > Mahalalel receives crown (841)
Mahalalel dies (895) > Jared receives crown (831)
Jared still alive (526) > Enoch ascends to heaven (365)
Jared dies (962) > Methuselah (Enoch's firstborn) receives crown (737)
Methuselah still alive (963) > Lamech dies (777)
Methuselah dies (969) > Noah (Lamech's firstborn) receives crown (602)
Noah dies (950) > Shem receives crown (448)
Shem is then alive during the following generations of firstborns (plus other offspring):
Arphaxad > Shelah > Eber > Peleg > Reu > Serug > Nahor > Terah > Abram (Abraham)
By my extrapolation, Abram (the great, great, great, great, great, great grandson) was 7 when Noah dies and dominion was passed to Shem, but by this time they were 9 generations apart so they would've been complete strangers.
By the time Abram was 85-86 (living 10 years in Canaan and conquering the kings who enslaved Lot), Shem (527), Arphaxad (428), Shelah (394), and Eber (365) were still alive, outliving Peleg, Reu, Serug, Nahor, and Terah. But Shem was still the eldest firstborn from Adam so he was the Crown of Earth, without (living) mother or father or any known genealogical connection to Abram.
At this point I can see Shem having the "Priestly-king" authority (as "crown of earth" / "representative of God") to bless Abram...and Abram tithing to Shem as God's representative priest.
Shem was still alive (537) when Arphaxad dies (438); Abram is approx 96
Shem was still alive (566) when Shelah dies (433); Abraham is approx 125; Isaac (firstborn by faith) is 26
Shem finally dies at 600 > Eber receives crown (438); Abraham is approx 159; Isaac is 60
Eber (454) then outlives Abraham who dies at 175; Isaac is 76
And when Eber dies (464), the crown of earth (called the "birthright") passes straight to Isaac at 86...
Then Esau (Isaac's eldest twin son) sells his future "
birthright" to Jacob for pottage (God then hates Esau for rejecting his "birthright")...and later at Jacob's death, Jacob splits his "birthright" between
Judah (
the crown/dominion) and
Joseph's sons (
the blessing/fruitfulness).
Many years later, the Levitical Priesthood is established by God as a temporary ministration of the "breach in contract" only AFTER the covenant at Sinai was broken, while the
Melchizedek Priesthood was maintained through
Judah's crown (as David & Solomon showed; both serving as High Priest in their day, making sacrifices to God).
At least that's what I've pieced together from the records...