Genèse 14:18

And Malka Zadika, who was Shem bar Noah, the king of Yerushalem, came forth to meet Abram, and brought forth to him bread and wine; and in that time he ministered before Eloha Ilaha.

AND MELCHIZEDEK KING OF SALEM. This is Jerusalem, just as it is said, In Salem is set His tabernacle. In the days of Joshua, its king was also called Adoni-zedek. Since time immemorial the nations knew that this place, which was the choicest of all places, is in the center of the inhabited region. Or perhaps they knew of its superiority by tradition, i.e., that it is exactly opposite the Heavenly Sanctuary, where the Divine Glory of the Holy One, blessed be He, who is called Tzedek (Righteousness) abides., literally, “My king is righteousness.”
In Bereshith Rabbah [we find that Jerusalem is called Tzedek because] “this place makes its inhabitants righteous. And Melchizedek means ‘the lord of Zedek.’ Jerusalem is called Tzedek, as it is said, ‘Tzedek’ (Righteousness) lodged in it.”

הוציא לחם ויין, to all those returning from battle who were tired.

והוא כהן, it was appropriate therefore that he should intone blessings.

ומלכי צדק מלך שלם, the word שלם is another word for ירושלים, and we find it referred to as such in Psalms 76,3 ויהי בשלם סכו, “Salem became His abode.” It was customary to give the title Malki Tzedek or Adoni Tzedek, to kings of Jerusalem in those days, just as the kings of Egypt were called Pharaoh. Compare Joshua 10,1 where we find אדני צדק מלך ירושלים. Jerusalem traditionally represented the center of righteousness and peace. It is a place in which iniquity is less tolerated than anywhere else. This is why the Torah warned in Leviticus 18,28 that the Israelites should take care that the land not spew them out just as it had spewed out previous inhabitants because they had defiled it. The reason the Torah in Deuteronomy 31,16 uses the somewhat enigmatic expression אלוהי נכר הארץ, is that the kind of conduct by the people in that city that were spewed out, was a conduct that was irreconcilable (alien) with the very nature, spiritual climate, prevailing in, and being part of that holy city. The sons of Korach already recognised and extolled this spiritual climate of the very city when they said in Psalms 48,3 יפה נוף משוש כל הארץ הר ציון ירכתי צפון קרית מלך רב, “fair-crested, joy of all the earth, Mount Zion, summit of Tzafon, city of the great King.” In the Midrash quoted by Rashi, Malki Tzedek is identified as Shem, the son of Noach.

הוציא לחם ויין, seeing that these people came back from battle tired, he brought them bread and wine to strengthen and refresh them.

והוא כהן לאל עליון, he did not worship the stars or other celestial phenomena but only the supreme and invisible G’d. In this respect he was like Avram and those who had joined him.

ומלכי צדק מלך שלם הוציא לחם ויין, “and Malki Tzedek, King of Shalem, produced bread and wine, etc.” This was the custom in which soldiers returning from battle would be welcomed. They were naturally tired and worn out, and needed to replace spent energy by imbibing drink and consuming food. An alternate explanation: the reference to bread and wine, specifically, is a hint that just as that King was a priest, so priests of the Jewish people would perform temple service offering libations of wine and the showbreads to the Almighty on a regular basis. In this instance Malki Tzedek, up to now a priest of the supreme G–d, forfeited that honour when he blessed Avraham before first blessing the Almighty, Who had enabled Avraham to become the victor. When David, in Psalms 110,4 said: נשבע ה' ולא ינחם אתה כהן לעולם על דברתי מלכי צדק, “the Lord has sworn and will not relent, ’you are a priest forever, a rightful priest by My decree,’” he referred to the incident in our verse which cost Malki Tzedek his priesthood. [All the people killed in that battle were actually killed by G–d, as a priest who has killed a human being is forthwith denied the right to perform service in the Temple. Abraham certainly had represented the priesthood in his age. He was “officially” appointed as such after this incident. Ed.] According to the Talmud in Nedarim 32, quoted earlier, this is all derived from the word כהן, “a priest,” which our sages understand to mean that his offspring were not priests.