Hebrews 7

Paul as taught in chapter 5, Paul again teaches that there is priesthood authority whereby -

"...no man taketh this honour unto himself, but he that is called of God, as was Aaron." (New Testament | Hebrews 5:4)

Paul teaches that there is the two priesthoods, the Melchizedek priesthood and the Aaronic or Levitical priesthood. Teaching of the lessor priesthood, Paul writes -

"If therefore perfection were by the Levitical priesthood, (for under it the people received the law,) what further need was there that another priest should rise after the order of Melchisedec, and not be called after the order of Aaron?" (New Testament | Hebrews 7:11)

We learn that the priesthood held by Melchizedek is -

"Without father, without mother, without descent, having neither beginning of days, nor end of life..." (New Testament | Hebrews 7:3)

The office of high priest is of the higher priesthood or the Melchizedek priesthood of such the priesthood the Lord has maded an oath and covenants for -

"...this [priesthood] with an oath by him that said unto him, The Lord sware and will not repent, Thou art a priest for ever after the order of Melchisedec..." (New Testament | Hebrews 7:21)

We learn more of this Oath and Covenant that beloneth to the priesthood in modern revelation -

"For whoso is faithful unto the obtaining these two priesthoods of which I have spoken, and the magnifying their calling, are sanctified by the Spirit unto the renewing of their bodies.
34 They become the sons of Moses and of Aaron and the seed of Abraham, and the church and kingdom, and the elect of God.
35 And also all they who receive this priesthood receive me, saith the Lord;
36 For he that receiveth my servants receiveth me;
37 And he that receiveth me receiveth my Father;
38 And he that receiveth my Father receiveth my Father's kingdom; therefore all that my Father hath shall be given unto him.
39 And this is according to the oath and covenant which belongeth to the priesthood.
40 Therefore, all those who receive the priesthood, receive this oath and covenant of my Father, which he cannot break, neither can it be moved." (Doctrine and Covenants | Section 84:33 - 40)

What a grand oath and covenant to those that magnify their calling within these priesthoods.