Hebrews 7:21 - Matthew Poole's English Annotations on the Holy Bible

For those priests were made without an oath; those priests of Aaron's order were selected, instituted, consecrated, without any oath mentioned by Moses, who did all exactly as the Lord commanded him, Exodus 40:16. God gave only command for it, and made their priesthood but a temporary and passing honour and office, which he might alter when he would. But this with an oath by him that said unto him: The Lord sware and will not repent: but he, or Jesus, was made a Priest after Melchisedec's order, by an oath of God his Father, speaking to him, as is recorded by David, Psalms 110:4. The Lord Jehovah the Father, sware unto his Son the Lord Messiah, lifting his hand, and saying: I live for ever, Deuteronomy 32:40, when he ascended and sat down on the right hand of the Majesty in the heavens, solemnly by this oath ratifying and confirming him in this office; and that he would not repent, i.e. change, or alter, or retract what he swore to him, there being no need of any other, he so effectually performing the work of it, that all that God bestows upon his by him, are gifts not to be repented of even eternal life and salvation. Thou art a Priest for ever after the order of Melchisedec: that which the oath ratified was, that Christ should be God's only and eternal Priest, who was to have no sharer with him in the priesthood, and no end of it; taking away from himself by oath any power to make Christ no priest, or take away his office at will and pleasure, as he did Aaron s; hereby honouring his Son, and highly gratifying sinners by giving them such a royal High Priest, who should effectually manage all their concernments with him for ever.