Matthew Poole and continuators on Hebrews 5:6

6 As he saith also in another place, Thou art a priest for ever , after the order of Melchisedec .

The Spirit proves his Call and Investiture into this Office, its Confirmation to him for ever by another Testimony of the Father about it, penn'd by David, Psalm 110.4. and ratified to be so by the Lord himself, Matth. 22.41 to 45. that he as Man was David's Son, as God-Man, David's Lord, and the grand Officer to atone God by his Sacrifice for Sinners, and to interceed for him. By this Word of God to him was he invested with the most glorious Priesthood, and setled in that which he must execute for ever, ch. 7.24. having no Successor in it,

which Order was a singular and most excellent one; such as Aaron's did but imperfectly shadow to us. It was a Royal Priesthood God installed him in, such as was Melchisedeck's, largely described, ch. 7. This was by God the Father revealed to David, and prophesied by him to the Church, but actually fulfilled as to Proclamation and Inauguration at his Ascension into the Holy of Holiest in Heaven, where he actually in the Flesh doth officiate and minister in it.

Marginal references: Chap. 7.17.