Adam Clarke on Hebrews 3:5

As a servant - The fidelity of Moses was the fidelity of a servant; he was not the framer of that Church or house; he was employed, under God, to arrange and order it: he was steward to the Builder and Owner.

For a testimony of those things - Every ordinance under the law was typical; every thing bore a testimony to the things which were to be spoken after; i.e. to Jesus Christ, his suffering, death, and the glory which should follow; and to his Gospel in all its parts. The faithfulness of Moses consisted in his scrupulous attention to every ordinance of God; his framing every thing according to the pattern showed him by the Lord; and his referring all to that Christ of whom he spoke as the prophet who should come after him, and should be raised up from among themselves; whom they should attentively hear and obey, on pain of being cut off from being the people of the Lord. Hence our Lord told the Jews, Joh 5:46 : If ye had believed Moses, ye would have believed me, for he wrote of me; "namely;" says Dr. Macknight, "in the figures, but especially in the prophecies, of the law, where the Gospel dispensation, the coming of its Author, and his character as Messiah, are all described with a precision which adds the greatest lustre of evidence to Jesus and to his Gospel."