How was Melchizedek the high priest of both Jews and Gentiles?
Melchizedek served as priest of the Most High God, a role indicating a predominantly Gentile scope, yet he was also high priest of the Jew. Therefore he was high priest of all, not merely one group, and Christ's high priesthood is after Melchizedek's order, not Aaron's.
Les explains that Melchizedek was king of Salem, the city of peace, and priest of the Most High God. There had to be a high priest not only for the Jew but for the whole Gentile world. Melchizedek's title as priest of the Most High God indicates predominantly the Gentile aspect, yet he was also to be high priest of the Jew—not just Gentile, but of all. Les concludes that Melchizedek was the high priest of all, and Christ's high priesthood is not after the order of Aaron but after the order of Melchizedek. The context also notes that Christ sprang from Judah, not Levi, so He could not be a priest under the Mosaic priesthood, which required Levites.
More Questions
Why was there a need for Melchizedek to be a priest before the Levitical priesthood?
There had to be a high priest not just for the Jew but for the whole Gentile world as well, and Melchizedek was that priest of the Most High God, serving as high priest of both Jews and Gentiles.