Hebrews 7:21

“For these priests were made without an oath, etc..] The priests of the tribe of Levi, and of the order of Aaron, were installed into their office, and invested with it, without an oath; no mention is made of any when Aaron and his sons were put into it in Moses's time; nor was any used afterwards, neither by God, nor by the priests, nor by the people; it is true indeed that after the sect of the Sadducees arose, the high priest on the day of atonement, was obliged to take an oath that he would not change any of the customs of the day f115; but then this regarded not his investiture, but the execution of his office; and was an oath of his and not of the Lord's, which is here designed: but…”

“Translate in the Greek order, "For they indeed (the existing legal priests) without the (solemn) promise on oath (so the Greek [TITTMANN]) are made priests." by him--God. unto him--the Lord, the Son of God (Psa 110:1). not repent--never change His purpose. after the order of Melchisedec--omitted in some oldest manuscripts, contained in others.”

“For those priests were made without an oath - The Levitical priests were set apart and consecrated without their office being confirmed to them by an oath on the part of God. They received it by regular descent, and when they arrived at a suitable age they entered on it of course. Jesus received his office by special appointment, and it was secured to him by an oath. The word rendered “oath” is in the margin “swearing of an oath.” This is the proper meaning of the Greek word, but the sense is not materially varied. But this with an oath - This priest, the Lord Jesus, became a priest in virtue of an oath. The Lord sware - see the note at Hebrews 6:13. The reference here is to Psalms 110:4.…”

“The Lord sware and will not repent - Hence also it appears, that his is an unchangeable priesthood. Hebrews 7:22”

“an oath: or, swearing of an oath The Lord: Heb 7:17; Psa 110:4 sware: Heb 6:16-18 Hebrews 7:22”