A priest forever in Melchizedek's order
Hebrews repeatedly quotes Psalm 110's oath while explaining that Jesus is a priest forever in the order of Melchizedek.
So also Christ did not take upon Himself the glory of becoming a high priest, but He was called by the One who said to Him: “You are My Son; today I have become Your Father.” And in another passage God says: “You are a priest forever in the order of Melchizedek.”
We have this hope as an anchor for the soul, firm and secure. It enters the inner sanctuary behind the curtain, where Jesus our forerunner has entered on our behalf. He has become a high priest forever in the order of Melchizedek.
And this point is even more clear if another priest like Melchizedek appears, one who has become a priest not by a law of succession, but by the power of an indestructible life. For it is testified: “You are a priest forever in the order of Melchizedek.”
And none of this happened without an oath. For others became priests without an oath, but Jesus became a priest with an oath by the One who said to Him: “The Lord has sworn and will not change His mind: ‘You are a priest forever.’” Because of this oath, Jesus has become the guarantee of a better covenant.
Psalm 110:4 declares with an unchanging oath, "You are a priest forever in the order of Melchizedek"; Hebrews 5:6 introduces the declaration, Hebrews 6:20 anticipates its fuller treatment, and Hebrews 7:17 and 21 quote it while contrasting Jesus' enduring priesthood with mortal priests.