Psalms 110:4 - Joseph Benson’s Commentary on the Old and New Testaments

The Lord hath sworn He did not swear in constituting the Aaronical priesthood, Hebrews 7:21, but did here, that it might appear to all mankind that this priesthood was made irrevocable, which that of Aaron was not. Thou art a priest As well as a king. Those offices, which were divided before between two families, are both united and invested in thee, both being absolutely necessary for the establishment of thy kingdom, which is of a nature totally different from the kingdoms of this world, being spiritual and heavenly, and therefore requires such a king as is also a minister of holy things. For ever Not to be interrupted or translated to another person, as the priesthood of Aaron was upon the death of the priest, but to be continued to thee for ever. After the order of Melchisedek Or, after the manner, &c.; that is, so as he was a priest and also a king, and both without any successor and without end, in the sense intended, Hebrews 7:3.