Why is Jesus not just the high priest of the Jews?
Christ is a high priest after the order of Melchizedek, not after Aaron. Melchizedek was not a priest under the Law or of the tribe of Levi, but a priest of all, covering the whole spectrum of the human race. Therefore Jesus is not merely the high priest of the Jew, but high priest of whomever we might be.
Les explains that Abraham's confrontation with Melchizedek came after Abraham had defeated the kings that had overrun Sodom. Melchizedek came from Salem, or Shalom, the city of peace, which is Jerusalem. He was a high priest forever, and Christ is a high priest after the order of Melchizedek, not after Aaron, because Melchizedek was not a priest under the Law and was not of the tribe of Levi. Rather, he was a priest of all and covered the whole spectrum of the human race. Therefore Christ's priesthood is not limited to the Jew, but extends to whomever we might be. Les also connects this to Hebrews 9, where Christ became a high priest of good things to come by a greater and more perfect tabernacle.