John Chrysostom

"For a testimony of those things which shall be spoken." What meanest thou? Doth God receive the witness of man? Yes, certainly. For if He call to witness heaven and earth and hills (saying by the prophet, "Hear, O heaven, and give ear, O earth, for the Lord hath spoken" - and "Hear ye ravines, foundations of the earth, for the Lord hath a controversy with His people"), much more men; that is, that they may be witnesses, when themselves the Jews are shameless.

Homily on Hebrews 5 ↗

Theophylact of Ohrid

Behold, he shows yet another superiority: Moses was faithful as a servant, appointed to convey the Lord's commands to the other servants and to be a witness before God at the judgment concerning what was told to them. For if he calls heaven and earth as witnesses, as for example: "Give ear, O heavens, and hear, O earth" (Deut. 32:1; cf. Isa. 1:2), and the valleys, as for example: "Hear, O strong foundations of the earth" (Mic. 6:2), then how much more a man.

Commentary on Hebrews ↗

Oecumenius

"And Moses indeed was faithful in all his house." Then Paul speaks of another distinction between Christ and Moses, that Moses was faithful as a servant, while the Christ was a genuine Son and of the same essence. For since he called Christ faithful above, and also Moses faithful, he explains the distinction of the faithful. "for a testimony of those things which were to be spoken." To be a witness of the things spoken by God to the Jews, so that they may not be caught in the time of judgment. …

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

165. – Then (v. 5) he prefers Christ to Moses in regard to their state. In regard to this he does two things: first, he states his reason; secondly, he explains it (v. 6b). 166. – His reasoning is this: It is obvious that the master is of more value in his own house than a servant in the master's house. But Moses was faithful as a servant in the house of his master, but Christ as the master in His own house, therefore, etc. …

Commentary on Hebrews ↗