Jamieson, Fausset and Brown on Hebrews 5:6

6. He is here called simply "Priest"; in He 5:5, "High Priest." He is a Priest absolutely, because He stands alone in that character without an equal. He is "High Priest" in respect of the Aaronic type, and also in respect to us, whom He has made priests by throwing open to us access to God [Bengel]. "The order of Melchisedec" is explained in He 7:15, "the similitude of Melchisedec." The priesthood is similarly combined with His kingly office in Zec 6:13. Melchisedec was at once man, priest, and king. Paul's selecting as the type of Christ one not of the stock of Abraham, on which the Jews prided themselves, is an intimation of Messianic universalism.