The allusion to Psalm 110:4 serves to reconfirm God’s proclamation of Jesus as high priest (cf. Hebrews 5:6) and to connect Jesus’ priesthood to his work of salvation. He who as the Perfecter of perpetual salvation (Hebrews 5:9) completely fulfills the Aaronic priesthood, could accomplish this as a high priest in the manner of Melchizedek.1,2
10 being designated by God a high priest after the order of Melchizedek.