John Calvin Commentary Hebrews 3:5

"And Moses indeed was faithful in all his house as a servant, for a testimony of those things which were afterward to be spoken;" — Hebrews 3:5 (ASV)

And Moses truly was faithful in all his house, as a servant. The second difference is that a doctrine was committed to Moses to which he, in common with others, was to submit. But Christ, though he took on the form of a servant, is still Master and Lord, to whom all ought to be subject. For, as we found in Hebrews 1:2, he has been made heir of all things.

For a testimony of those things which were to be spoken after, or which were afterwards to be said or declared. I explain this simply in this way—that Moses, while a herald of that doctrine which was to be published for a time to the ancient people, at the same time gave testimony to the Gospel, the publication of which was not yet to be made. For it is undoubtedly clear that the end and completion of the Law is that perfection of wisdom contained in the Gospel. This explanation seems to be consistent with the future tense of the participle. The meaning indeed is that Moses faithfully delivered to the people what the Lord had committed to him, but limits were prescribed to him which he was not permitted to pass. God formerly spoke at different times and in various ways by the prophets, but he deferred the complete revelation of the Gospel to the fullness of time.