Hebrews 3:5

Moses was faithful ...as a servant - Not as the head of the dispensation; not as having originated it; but as in the employ and under the direction of its great Founder and Author - the Messiah. As such a servant he deserves all the honor for fidelity which has ever been claimed for him, but it cannot be the honor which is due to him who is at the head of the family or house. Paul “assumed” that Moses was a “servant,” and argued on that supposition, without attempting to prove it, because it was so often affirmed in the Old Testament, and must have been conceded by all the Jews. In numerous instances he is spoken of as “the servant of the Lord;” see Joshua 1:1; Jos 9:24; 1 Chronicles 6:49; 2 Chronicles 24:9; Nehemiah 10:29; Daniel 9:11; Exodus 14:31; 1 Kings 8:56; Psalms 105:26. As this point was undisputed, it was only necessary to show that the Messiah was superior to a “servant,” in order to make the argument clear.

For a testimony - To bear witness to those truths which were to be revealed; that is, he was the instrument of the divine communications to the people, or the medium by which God made his will known. He did not originate the truths himself; but he was the mere medium by which God made known his truth to his people - a servant whom He employed to make his will known. The word after here is not necessary in order to a just translation of this passage, and obscures the sense. It does not mean that he was a witness of those truths which were to be spoken “subsequently” to his time under another dispensation, nor those truths which the apostle proposed to consider in another part of the Epistle, as Doddridge supposes; but it means merely that Moses stood forth as a public witness of the truths which God designed to reveal, or which were to be spoken. God did not speak to his people “directly,” and face to face, but he spoke through Moses as an organ, or medium. The sense is, Moses was a mere servant of God to communicate his will to man.