For the source text click/tap here: Nedarim 32
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Rabbi Zecharyah said in the name of Rabbi Yishmael: The Holy One, Blessed be He, wanted to have the Kehunah descend from Shem (the son of Noach), as it is stated: and he was a priest of God, the Highest. However, once Shem recited a blessing of Avraham before the blessing of God, He decided that the Kehunah should emerge from Avraham. This is as it states: And he (Malkitzedek, who was Shem) blessed him (Avraham), and he said: Blessed is Avram to God, the Most high, Maker of heaven and earth, and blessed is God, the Most High. Avram said to him: Is it appropriate to mention first a blessing of the servant before the blessing of his Master?
Immediately, He gave the kehunah to Avraham, as it states: The word of Hashem to my master: Until I make your enemies into a footstool for your feet, and afterwards it is written Hashem has sworn and will not relent: You shall be a priest forever, for you are a king of righteousness (al divrei Malkitzedek).
This was on account of the words of Malkitzedek. And this explains that which is written: And he (Shem) was a priest of God, the Most high. This implies that he (Shem) was a kohen, but his children would not be kohanim.
We explore the curious history of this apparent minor character who exercised the rabbis and the early church fathers because of the historical lineage of the priesthood.