John Calvin Commentary Hebrews 7:8

"And here men that die receive tithes; but there one, of whom it is witnessed that he liveth." — Hebrews 7:8 (ASV)

Of whom it is witnessed that he liveth. He takes the silence concerning his death, as I have said, as evidence of his life. This would not indeed apply to others, but for Melchizedek it ought rightly to be regarded this way, since 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 further than what we read in Scripture.

But we are not from this to conclude that the man who met Abraham is still 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 Melchizedek’s priesthood was to be perpetual, while that of the Levites was temporary.

For he reasons as follows: those to whom the Law assigns tithes are dying men, by which it was indicated that the priesthood would eventually be abolished as their lives came to an end; but the Scripture makes no mention of Melchizedek’s death when it relates that tithes were paid to him, so the authority of his priesthood is limited by no time; on the contrary, an indication of perpetuity is given.

And this is added for this purpose: to prevent a later law, as is usual, from seeming to take away from the authority of a former law. For it might have been otherwise objected that the right which Melchizedek previously possessed is now null and void, 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 were mortal; but that Melchizedek, because he is immortal, retains even to the end what was once given to him by God.