Pursuing the subject of Jesus' priesthood still further, the author of Hebrews gives his own explanations concerning the necessity for a new type of priesthood to replace the older one associated with the tribe of Levi. Again he regards the question of duration as important. The office of priest was hereditary among the Levites; when a priest died, it was necessary for him to be replaced by another, whose right to the office was determined by whether he was a descendant from that particular tribe. Because it was generally recognized that Jesus came from the tribe of Judah, one that was not designated as a tribe from which the priests were chosen, it could be inferred that Jesus' right to the priesthood was based not on physical descent but "by the power of an endless life." Furthermore, we are told that Jesus' appointment to the priesthood was confirmed by an oath, whereas no such oath was used in the appointment of any of the Levitical priests. The author finds support in his interpretation of a passage from Psalms 110, which reads, "The Lord has sworn and will not change his mind: 'You are a priest forever.'" Assuming that the psalmist was referring to Jesus, the statement gives added support to the author's conviction concerning the superiority of the Christian priesthood of Jesus.
This conviction is illustrated again in the assertion that the services performed by the Levitical priests were a part of the system referred to as the Old Covenant. In contrast, the priesthood of Jesus belongs to the New Covenant. Mention of these two covenants is made in a reference to the passage in the Book of Jeremiah in which the prophet contrasts the idea of obedience to a set of external laws with the type of conduct that is motivated by the right desires and purposes within an individual. The former constitutes the basis of the Old Covenant, the latter the basis for the New Covenant. The author of Hebrews tells us that the imperfections of the Levitical priesthood were due, at least in part, to the attempt made to regulate conduct according to the requirements of the Mosaic Law. The failure of this attempt was one of the reasons that made a new and different type of priesthood necessary, which, the author holds, was accomplished in the priesthood of Jesus in accordance with which Jesus became a minister of the New Covenant.
