Hebrews 7:8

8.Of whom it is witnessed that he liveth. He takes the silence respecting his death, as I have said, as an evidence of his life. This would not indeed hold as to others, but as to Melchisedec it ought rightly to be so regarded, inasmuch as he was a type of Christ. For as the spiritual kingdom and priesthood of Christ are spoken of here, there is no place left for human conjectures; nor is it lawful for us to seek to know anything farther than what we read in Scripture. But we are not hence to conclude that the man who met Abraham is yet alive, as some have childishly thought, for this is to be applied to the other person whom he represented, even the Son of God. And by these words the Apostle intended to show, that the dignity of Melchisedec’s priesthood was to be perpetual, while that of the Levites was temporary. (116)

For he thus reasons, — those to whom the Law assigns tithes are dying men; by which it was indicated that the priesthood would some time be abrogated, as their life came to an end: but the Scripture makes no mention of the death of Melchisedec, when it relates that tithes were paid to him; so the authority of his priesthood is limited by no time, but on the contrary there is given an indication of perpetuity. And this is added for this purpose, lest a posterior law, as it is usual, should seem to take away from the authority of a former law. For it might have been otherwise objected and said, that the right which Melchisedec formerly possessed is now void and null, because God had introduced another law by Moses, by which he transferred the right to the Levites. But the Apostle anticipates this objection by saying, that tithes were paid to the Levites only for a time, because they did not live; but that Melchisedec, because he is immortal, retains even to the end what was once given to him by God.