Psalms 89:34-36

89:34 I will not break my covenant

or go back on what I promised.

89:35 Once and for all I have vowed by my own holiness,

I will never deceive David.

89:36 His dynasty will last forever.

His throne will endure before me, like the sun,

Hebrews 5:6

5:6 as also in another place God says, “You are a priest forever in the order of Melchizedek.”

Hebrews 6:13-18

6:13 Now when God made his promise to Abraham, since he could swear by no one greater, he swore by himself, 6:14 saying, “Surely I will bless you greatly and multiply your descendants abundantly.” 6:15 And so by persevering, Abraham inherited the promise. 6:16 For people swear by something greater than themselves, and the oath serves as a confirmation to end all dispute. 6:17 In the same way God wanted to demonstrate more clearly to the heirs of the promise that his purpose was unchangeable, and so he intervened with an oath, 6:18 so that we who have found refuge in him may find strong encouragement to hold fast to the hope set before us through two unchangeable things, since it is impossible for God to lie.

Hebrews 7:28

7:28 For the law appoints as high priests men subject to weakness, but the word of solemn affirmation that came after the law appoints a son made perfect forever.