And King Melchizedek of Salem brought out bread and wine;
he was priest of God Most High.
He blessed him and said,
“Blessed be Abram by God Most High,
maker of heaven and earth;
and blessed be God Most High,
who has delivered your enemies into your hand!”
And Abram gave him one-tenth of everything.
-The Book of Genesis, Chapter 14, verses 18-20, New Revised Standard Version
This brief set of verses takes place at an interesting point: most of Genesis 14 centers around a war narrative of various kings and armies going to battle, of Abram going to battle and regaining the people stolen from him and such. King Melchizedek, a priest-king, appears just before another figure, the King of Sodom, and interrupts the narrative.
The name "Melchizedek" sounds like "King of Righteousness" in Hebrew.
Salem here probably refers to Jerusalem, and it's possible this story is meant to mark an ancient precedent for Jerusalem being the holy city God.
"God Most High" is "El Elyon" in Hebrew. "Elyon" is often added to the names of deities in this era, and it's probably a way of denoting majesty.
Interesting from the Christian perspective is that the ordination rites in the Apostolic Succession feature a phrase along the lines of, "Thou art forever a priest after the order of Melchizedek."
Also interesting is that Melchizedek brings forth bread and wine, possibly a precedent to later rituals involving bread and wine, including the Shabbat and the Holy Eucharist.
Abraham giving a tenth of what he owned seems to lay a precedent for tithing.