Without getting tied up in the conjecture of men's thinking about Melchisedec it is best to recognize that as far as the record of Scripture goes, Melchisedec shows up ministering to Abraham and then disappears. Also, in verses of the Bible, Melchisedec is without father and mother, having no recorded genealogy. He does not appear as the son of any man which could link him to an office of priesthood. In other words, there is no record of him having a physical lineage that qualifies him as a candidate to hold a priest's office.
The importance that Scripture places on Melchisedec is not upon his person, but upon his type of priesthood. And that is what Hebrews 7 does; it is not speaking about the person of Jesus Christ, for that was covered in chapters 1 and 2, but what is presented in Hebrews 7 is the priesthood of Christ. The order of this new priesthood, whereby Israel can now draw nigh unto God, is perfect as its establishment, is eternal, forever, and abideth continually.
From: A Dispensational Survey of the Epistles to the Hebrews