John Chrysostom

"For He of whom these things are spoken pertained to another tribe, of which no man gave attendance at the altar. For it is evident that our Lord sprang out of Judah, of which tribe Moses spake nothing concerning priests." I myself also say it, I know that this tribe of Judah had nothing of priesthood. For there is a transferring. And observe the mystery. …

Homily on Hebrews 13 ↗

Theophylact of Ohrid

He shows how the priesthood was changed according to tribe, and says that Christ ("of Whom" (ἐφ᾿ ὃν) instead of τον, περί ου – that is, "to Whom" the priesthood passed over) – is from another tribe, namely from Judah. "From which," namely from the tribe of Judah, "no one approached," that is, no one stood before the altar and performed priestly duties.

Commentary on Hebrews ↗

Oecumenius

"For he of whom these things are spoken." It is certainly said concerning Christ, "You are a priest forever according to the order of Melchizedek." (Heb. 7:17) "belongs to another tribe." And so great, he says, is the change and interchange of both the law and the old covenant, that even of that tribe, which administered the old priesthood, there has been a change. For Christ is of another tribe, not of Levi but of Judah, which tribe, neither at any time served as a priest at the altar.

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

355. – Then when he says, he of whom these things are spoken belonged to another tribe, he clarifies what he had said: first, that the priesthood was transferred; secondly, he explains this (v. 14). 356. – He says, therefore: It has been stated that the priesthood was transferred, because the one to whom the prophet said, 'You are a priest forever,' is of another tribe, namely, of Judah and not of Levi, as is clear from Matthew (1:3), from which tribe, namely, Judah, no one has served the altar. …

Commentary on Hebrews ↗