John Chrysostom

"And it is yet far more evident, if after the similitude of Melchisedec there ariseth another priest." What is evident? The interval between the two priesthoods, the difference; how much superior He is "who was made not according to the law of a carnal commandment." (Who? Melchisedec? Nay; but Christ.)

Homily on Hebrews 13 ↗

Theophylact of Ohrid

What is "more clearly seen"? The middle ground between one priesthood and the other. Or that the change of priesthood and covenant is revealed not only from the fact that the priest arises from a different tribe, and by no means from that of Levi, but also, let us say this, is more fully revealed from this as well: that "after the likeness of Melchizedek" and so on. This means that after the order of Melchizedek a priest arises.

Commentary on Hebrews ↗

Oecumenius

And not only, he says, it is clear from this that the priesthood and the covenant have been changed from that of another tribe, and not from Levi, that the risen priest is from, but also it is evident from this more abundantly. Of which? From that according to the order of Melchizedek, and not according to the former priest Aaron, Christ became a priest. …

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

358. – Then when he says, This becomes even more evident, because above he had presented one reason to prove that Christ's priesthood is preferred over the Levitical and does away with it; he now presents the other reason, in which he shows why it is done away with and changed. To do this he makes use of a conditional, in the first of which he lays down two antecedents, and in the second two consequents. In regard to the first he does two things: first, he lays down those antecedents; secondly, he clarifies what he has said (v. 17).

Commentary on Hebrews ↗