Hebrews 7:4-6, 9-12

Now pause and consider how great this man was to whom Abraham, the patriarch, gave a tenth of the spoils. It is true that those descendants of Levi who are charged with the priestly office are commanded in the Law to collect tithes from the people—which means, from their kinsmen—though these have descended from Abraham. But this person [Melchizedek] who is not from their Levitical ancestry received tithes from Abraham and blessed him who possessed the promises [of God].

A person might even say that Levi [the father of the priestly tribe] himself, who received tithes, paid tithes through Abraham [the father of all Israel and of all who believe], for Levi was still in the loins (unborn) of his forefather [Abraham] when Melchizedek met him (Abraham). Now if perfection [a perfect fellowship between God and the worshiper] had been attained through the Levitical priesthood (for under it the people were given the Law) what further need was there for another and different kind of priest to arise, one in the manner of Melchizedek, rather than one appointed to the order of Aaron? For when there is a change in the priesthood, there is of necessity a change of the law [concerning the priesthood] as well.