Ps 110:4 The Lord has sworn and will not relent, “You are a priest forever according to the order of Melchizedek.”

This verse is from David but repeated a few times in Hebrews.
There is much to understand about this verse, which applies to Christ as the priest according to the order of Melchizedek.
According to Jewish tradition, Melchizedek is Shem bar Noah or Shem, son of Noah.
This would make him Abraham’s g-g-g-g-g-g-great grandfather.
Yes, he was still alive when Abram came back from rescuing Lot (Gen 14) and Melchizedek did something for the first time – Melchizedek king of Salem brought out bread and wine. Gen 14:18
Melchizedek brought out Communion.
Jesus, also, did something for the first time – Jesus took bread, blessed and broke it (Matt 26:26) and He took the cup, and gave thanks, and gave it to them. (Matt 26:27)
Jesus established the Lord’s Supper (Communion) in the middle of the Passover meal.
Melchizedek, an ancestor of Jesus Christ, gave the “body and the blood” to Abram (Abraham), also an ancestor of Jesus Christ.
Both did something different, Melchizedek in the Old Testament and Jesus in the New.
As I said in the beginning, there is much more to the comparison but I just thought I’d leave that with you for now.