Theophylact of Ohrid

And here he shows the advantage that Christ has in comparison with the high priests according to the law, and says that there are many priests there, because they are mortal; but here there is one, because He is immortal.

Commentary on Hebrews ↗

Oecumenius

"And indeed, there were many." And according to another, he says, the high priest is greater than us, not only because of the oath, but also because he is immortal. "because they were prevented by death." For it is through death and change, namely of the priest.

The Pseudo-Oecumenian Catena on Hebrews ↗

Cosmas Indicopleustes

For the high priest being eternal, of necessity the salvation also and the Tabernacle are eternal, in accordance again with what is written: And they indeed are many priests, because that by death they are hindered from continuing: but he, because he abideth for ever, hath his priesthood unchangeable. Wherefore also, he is able to save to the uttermost them that draw near unto God through him, seeing that he always liveth to make intercession for them. …

The Christian Topography, Book 7 ↗

Thomas Aquinas

367. – Then when he says, the former priests were many in number, he uses another clause stated in the authority: You are a priest forever. In regard to this he does two things: first, he shows why this phrase forever, is used; secondly, from this he shows that the priesthood of Christ has greater efficacy than the priesthood of the Old Testament (v. 25).

Commentary on Hebrews ↗