14 For that He is Jesus the Jews allow, but not further that He is Christ. Therefore says the Apostle, Who is the liar, but he that denies that Jesus is the Christ? But Christ is a High Priest, whose priesthood passes not to another, neither having begun His Priesthood in time, nor having any successor in His High-Priesthood: as you heard on the Lord's day, when we were discoursing in the congregation on the phrase, After the Order of Melchizedek. He received not the High-Priesthood from bodily succession, nor was He anointed with oil prepared by man, but before all ages by the Father; and He so far excels the others as with an oath He is made Priest: For they are priests without an oath, but He with an oath by Him that said, The Lord swore, and will not repent. The mere purpose of the Father was sufficient for surety: but the mode of assurance is twofold, namely that with the purpose there follows the oath also, that by two immutable things, in which it was impossible for God to lie, we might have strong encouragement for our faith, who receive Christ Jesus as the Son of God.
Source: Catechetical Lectures (New Advent)