John Calvin Commentary Deuteronomy 17:4

"and it be told thee, and thou hast heard of it, then shalt thou inquire diligently; and, behold, if it be true, and the thing certain, that such abomination is wrought in Israel," — Deuteronomy 17:4 (ASV)

Then three shall inquire³⁰⁷ diligently. Although this moderation here refers only to the present matter, it should always be maintained in judicial proceedings, lest innocent persons be treated with undue severity.

Again, we must remember what I have said elsewhere: judges are not only restrained here from precipitate condemnation but are also urged to take care not to overlook, through idleness or negligence, anything that needed to be known. For they often fail in their duty because they willfully connive at guilt; and thus, what would be manifest if they would take the trouble to inquire more diligently does not come to light. God, then, would not have them slumber or ignore sinister reports, but rather inquire diligently into matters that may have come to their ears, so that no crime remains unpunished.

The same is true concerning witnesses. For while it would be unjust to pronounce sentence based on the testimony of one man, still, if two or three will not suffice, there would be no end to litigation. Fittingly, then, God has prescribed to judges both that they shall not be rashly credulous and yet that they shall be content with the lawful number of witnesses. However, this point will be treated more extensively elsewhere when commenting on both the Sixth and Ninth Commandments.

³⁰⁷ A.V., translates this word in the past tense, “and translates this word in the past tense, “and hast inquired."inquired."