Vanhoye argues that the innovative feature of the Letter to the Hebrews is the exploration of the identity of Christ in terms of priesthood. The first part of the letter (1:5-2:18) prepares us for the theme:
In Part One the preacher reminded his audience of Christ's paschal mystery in traditional terms. He first spoke to them about Christ's glorification (1,5-14), then about his passion which brought about that glorification (2,5-16). To express these two phases of the mystery he used some traditional biblical texts used by apostolic catechesis, many of which are linked with Davidic Messianism of the royal kind. When ending (2,17-18), the author shows that the same Christology can also be expressed in priestly categories and that there is no difficulty in passing from a royal to a priestly explanation of the mystery of Christ. In fact, in his glorification, Christ is proclaimed Son of God and in his passion he is shown as brother of mankind. Through his glorifying passion he entered with his human nature of flesh and blood into the heavenly intimacy of his Father and at the same time strengthened his fraternal links with us and made them indissoluble. Having brought these two relations to their perfection, he now finds himself established in a position of perfect mediator between God and us or, in other words, of «merciful and trustworthy high priest for relations with God» (2,17)."
Albert Vanhoye, A Different Priest: The Epistle to the Hebrews (transl. Leo Arnold; Miami: Convivium Press, 2011), 114.
Vanhoye then contrasts royal and priestly Christology, too sharply for my taste. He asks, "For Jesus to be able to be proclaimed Messiah-King was it necessary that he should undergo the worst suffering and humiliation? That does not seem evident" (p115). But the path through suffering to glory has been trod by David himself and the Psalter suggests to me that he has thereby laid down a pattern, not least in Psalm 22. Vanhoye, however, does not read Psalm 22 as royal-messianic. Maybe he sees no royal features in Isa 53 either. He also does not grant Davidic kings a mediating role between God and his people but this seems to me very much assumed in the historical narratives. In fact, there really is "no difficulty in passing from a royal to a priestly explanation of the mystery of Christ". The true king was always meant to be priestly.
