Melchizedek appears briefly in Genesis 14:18-20, but his theological shadow stretches over the entire New Testament. He met Abraham returning from victory, bringing bread and wine, blessed him, and received a tithe. Almost nothing else is recorded about him. But David picks up the name in Psalm 110: "You are a priest forever according to the order of Melchizedek." The writer of Hebrews builds an entire Christology from this (Hebrews 5-7). Why does it matter? In the Old Testament, priesthood was restricted to Aaron's line, kingship to Judah (after David). You could not be both. But Melchizedek was both — king AND priest — and predated both Aaron and David. Christ's priesthood is "according to the order of Melchizedek" because He is simultaneously King and Priest, because His priesthood is not by ancestry but "by the power of an endless life" (Hebrews 7:16), and because He is greater than Abraham. The mysterious figure in Genesis 14 was a deliberate shadow of Christ, the eternal Priest-King.
Genesis 14:18-20 — "Melchizedek king of Salem brought out bread and wine; he was the priest of God Most High. And he blessed him."
Psalm 110:4 — "The LORD has sworn and will not relent, "You are a priest forever according to the order of Melchizedek.""
Hebrews 7:1-3 — "For this Melchizedek, king of Salem, priest of the Most High God... without father, without mother, without genealogy, having neither beginning of days nor end of life, but made like the Son of God, remains a priest continually."