The Genesis account of Abraham's encounter with Melchizedek leaves us with powerful few details about his life, yet later on the author of Hebrews clearly uses his priestly work as a type of Christ, going so far as to compare their origins and rolls as priest. Genesis at least tells us that the man was a king and priest:
Genesis 14:17-18 (ESV)
17 After his return from the defeat of Chedorlaomer and the kings who were with him, the king of Sodom went out to meet him at the Valley of Shaveh (that is, the King's Valley). 18 And Melchizedek king of Salem brought out bread and wine. (He was priest of God Most High.)
Hence the introduction in Hebrews doesn't tell us anything we don't already know. In fact in seems fairly obvious at first that the author's source is in fact the Genesis account.
Hebrews 7:1-2 (ESV)
7 For this Melchizedek, king of Salem, priest of the Most High God, met Abraham returning from the slaughter of the kings and blessed him, 2 and to him Abraham apportioned a tenth part of everything. He is first, by translation of his name, king of righteousness, and then he is also king of Salem, that is, king of peace.
However when we get to verse 3, it seems like we get an extra fragment of information about the life of Melchizedek than cannot be plainly derived from the Genesis account.
Hebrews 7:3 (ESV)
3 He is without father or mother or genealogy, having neither beginning of days nor end of life, but resembling the Son of God he continues a priest forever.
True, he kind of just shows up on the stage and walks off again when his scene is over, but I wouldn't normally assume that because a narrative leaves off the genealogy of a character that that character didn't have parents or eventually a tombstone somewhere. Yet that seems to be the conclusion drawn by Auctor.
Besides one brief mention in Psalms I don't know of any other Scriptural sources for information on Melchizedek. Are there other sources predating Hebrews that share this understanding of the narrative? Would Auctor have had other sources (either textual or traditional) for his/her understanding of Melchizedek's nature1?
In other words, was this a doctrinal understanding shared by others of his time, or, if one is to believe the text to be authoritative, does this need to be understood as an instance of divine revelation that made this detail available to those who received the book? Asked yet another way, was this text the first instance of Melchizedek being identified as specifically not having a beginning or end and for being an 'eternal priest'2?
1 I am aware that some believe this to be a pre-Christ theophany. I am less interested in the interpretation of the Hebrews passage as I am in knowing what sources Auctor may have had on this subject when it was written.
2 ...as opposed to your 'garden variety priest' that will one day find a home six feet under some church garden.