Psalms 110:4

Hath sworn; which he did not in the Aaronical priesthood, Hebrews 7:21, but did it here, partly because the thing was new and strange, and might seem incredible, because God had already erected another, and that an everlasting priesthood, Numbers 25:13, and given it to Aaron and his posterity for ever, and therefore this needed all possible assurance; and partly that his priesthood might be established upon better promises, as is said, Hebrews 8:6, and made sure and irrevocable, and such that God neither could nor would repent of it, as it follows. Thou art, to wit, by my order and constitution; thou shalt be so, and I do hereby make thee so. 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 discharge of thine office, and for the establishment of thy kingdom, which is of another kind than the kingdoms of the world, spiritual and heavenly, and therefore needs such a King as is also a minister of holy things. This word plainly discovers that this Psalm cannot be understood of David, as some of the Jews would have it, but only of the Messias. And although this word cohen be sometimes used of a prince or great person in the state, as the Jews object, yet it cannot be so understood here, partly, because it signifies a priest in Genesis 14:18, from whence this expression is borrowed; partly, because that word is never used of a sovereign prince or king, (such as the Jews confess the Messiahs to be,) but only of inferior princes or ministers of state, as Genesis 41:45 2 Samuel 8:18; partly, because such an inconsiderable assertion would never have been ushered in by so solemn an oath, especially after far greater things had been said of him in the same kind, Psalms 110:1 and partly, because the Messias is called a Priest, Malachi 6:13; compare Jeremiah 23:21, Jeremiah 35:15,18. 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 Melchizedek, or, after the manner, &c.; 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.