John Calvin Commentary Deuteronomy 17:6

"At the mouth of two witnesses, or three witnesses, shall he that is to die be put to death; at the mouth of one witness he shall not be put to death." — Deuteronomy 17:6 (ASV)

As His severity in exacting punishment, where murder has been unquestionably committed, shows how highly God values human life, so the qualification we find here declares that He takes equal care for the preservation of innocent blood.

For since excessive credulity would often impel judges to condemn the innocent, He applies a remedy here to this evil, forbidding the crime to be punished unless proven by reliable testimony.

Although He has naturally inscribed this law on every heart, He willed that it be written down so that its observance among the Israelites might be more sacred.

For nothing is more dangerous than to expose people's lives to the word of a single individual; but where the concurring testimony of two or three is carefully weighed, any hidden falsehood is usually detected.

Therefore, lest anyone be rashly condemned and innocence thereby be oppressed by unsubstantiated conjectures, insufficient accusations, or unjust prejudices, God intervenes here and does not allow anyone to be harshly treated unless properly convicted.