Adam Clarke on Deuteronomy 17:4

If it be told thee - In a private way by any confidential person. And thou hast heard of it; so that it appears to be notorious, very likely to be true, and publicly scandalous. And hast inquired diligently - sought to find out the truth of the report by the most careful examination of persons reporting, circumstances of the case, etc. And, behold, it be true - the report is not founded on vague rumor, hearsay, or malice. And the thing certain - substantiated by the fullest evidence. Then shalt thou bring forth that man, Deu 17:5. As the charge of idolatry was the most solemn and awful that could be brought against an Israelite, because it affected his life, therefore God required that the charge should be substantiated by the most unequivocal facts, and the most competent witnesses. Hence all the precautions mentioned in the fourth verse must be carefully used, in order to arrive at so affecting and so awful a truth.