John Chrysostom

"And here men that die receive tithes: but there he of whom it is testified that he liveth." But lest we should say, Tell us, why goest thou so far back? He says, "And as I may so say" (and he did well in softening it) "Levi also who receiveth tithes payed tithes in Abraham." How? "For he was yet in his loins when Melchisedec met him," i.e. Levi was in him, although he was not yet born. And he said not the Levites but Levi. Hast thou seen the superiority? …

Homily on Hebrews 12 ↗

Theophylact of Ohrid

And another argument proving that Melchizedek is superior to the priests of the law. For "here," that is, in the law, those who receive tithes die; but "there," that is, in the case of Melchizedek, the one who received tithes is one of whom Scripture testifies that he "lives." For "You," it says, "are a priest forever according to the order of Melchizedek" (Ps. 109:4). As for the fact that Melchizedek lives forever, understand this in the same way as was said above, that is, that Scripture makes no mention of his death. …

Commentary on Hebrews ↗

Oecumenius

It shows that there was great excellence in Melchizedek, and therefore he received tithes, and it says: Even the Levites receive; but this is not great, for they die; who, then, envies the honor of one who is about to die? However, Melchizedek, even if he received testimony that he lives, received tithes. …

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

344. – Then when he says, Here tithes are received by mortal men, he shows the pre-eminence of the priesthood on the part of the priest by reason of his state. His reasoning is this: That is the more excellent which is not corrupted. But in the Levitical priesthood mortal men, i.e., who succeed by death, receive tithes; but there, in the priesthood of Melchizedek, by one of whom it is testified from the Scripture, that he lives, i.e., it makes no mention of his death, not because he did not die, but because he signifies a priesthood …

Commentary on Hebrews ↗