I always found the story of Melchizedek interesting. Judaism was, from my understanding, not formed yet but he was introduced as a priest and Abram tithed to him. Here are the relevant verses from Genesis 14:
18 Then Melchizedek king of Salem brought out bread and wine. He was priest of God Most High, 19 and he blessed Abram, saying,
“Blessed be Abram by God Most High, Creator of heaven and earth.
20 And praise be to God Most High, who delivered your enemies into your hand.”
Then Abram gave him a tenth of everything.
So, the relevant questions are:
Where was Salem? What religion was Melchizedek representing or the clergy of? And, most importantly, who is Melchizedek? Was the Old Testament the only ancient text to mention him? Is he important in Islam or other Middle Eastern religions?
The truth is, we don't really know who Malkizedeq is. It's a queer passage for sure, as the priesthood doesn't get established until 'chronologically' much later (although we kind of have to set chronology aside for a moment considering the Documentary Hypothesis and source and redaction critical approaches to the Pentateuch). He's mentioned nowhere else in the Hebrew Bible, although he makes an appearance in Hebrews in the NT where Jesus is claimed to be a 'priest according to the order of Malkizedeq.'
Traditionally, Salem is identified as Jerusalem, although off the top of my head I'm not entirely certain how much this can be supported by archaeological evidence.
As far as religion - this phrase is most telling:
לְאֵל עֶלְיוֹן קֹנֵה שָׁמַיִם וָאָרֶץ
This is the last portion of 14:19 - my own translation: "...by El Elyon, Creator of the heavens and the earth."
El Elyon is a combination of two different deity names in Israelite/Canaanite religion. El (= 'God') and Elyon (= 'Most High') appear here in apposition to one another, obviously referring to a single deity, this could be a function of a later manifestation of Israelite religion (perhaps one closer to full blown monotheism?). This is unlikely a reference to Yahweh (Klaus Westermann doesn't blatantly say this, but his commentary suggests it).
It should be noted that Genesis 14 is often regarded as one of the latest additions to the book of Genesis — dating to the Persian period if not later. The story involves a war remarkably void of the supernatural phenomena present in most Abraham stories, and it presents Melchizedek as an early (if not the earliest) high priest of El Elyon / Yahweh. The text also takes the opportunity to deliberately present the two names as belonging to the same deity, even though the distinction between El and Yahweh is maintained in some earlier biblical passages. This is an anachronism within the biblical narrative, since the name Yahweh is not supposed to be revealed until the burning bush theophany in Exodus. (And Exodus 6 explicitly says that Abraham did not know the name of Yahweh.)
The story of Melchizedek may have also been intended to establish the legitimacy of the emerging Jerusalem priesthood in the Persian period. Salem is usually assumed to be another name for Jerusalem.
Once established as a biblical character, Melchizedek grew rapidly in popularity in Hellenistic/Roman times. Psalm 110, possibly written in association with Simon Maccabeus, uses Melchizedek as an example of the ideal priest-king of Jerusalem (the very role Simon played).
Melchizedek appears as a heavenly or angelic figure in one of the Dead Sea Scrolls, 11QMelch, as the one who will judge the righteous and destroy Belial (the devil) in the last days. The New Testament book of Hebrews draws on these legends of Melchizedek as a heavenly figure and progenitor of the Jewish priesthood to legitimate Jesus as another heavenly priest.
Melchizedek also appears as the heavenly high priest of God in a Gnostic text called Melchizedek from the second or third century CE.